Optical structures providing dichroic effects

ABSTRACT

A document, product, or package, such as a banknote, passport or the like comprises structures having dichroic effects that change color with viewing angle in both transmission and reflection. Such structures can be useful as security features that counter the ability to effectively use counterfeit documents, products, packages, etc.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

This application claims the benefit of priority of U.S. ProvisionalApplication No. 62/568,711 filed on Oct. 5, 2017, the entire disclosureof which is expressly incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED R&D

This invention was made with government support under Contract No. TEPS16-34769 awarded by the Bureau of Engraving and Printing. The governmenthas certain rights in the invention.

TECHNICAL FIELD

The present application generally relates to thin interference opticalstructures, films, coatings and pigments for producing color in bothreflection mode and transmission mode. More specifically, thesestructures, films, coatings, and pigments exhibit large color shiftingproperties with changes in both reflection and transmission potentiallywith a change in the angle of incidence or the viewing angle.

DESCRIPTION OF THE RELATED TECHNOLOGY

Color shifting features can be used as a security device (for example,on a banknote) to prevent counterfeiting. The color shifting effectproduced by the color shifting materials can be easy for the commonperson to observe. The color shifting effect produced by the colorshifting features, however, can be impractical to recreate usingcounterfeit copies produced by color copiers, printers and/orphotographic equipment. Color copiers, printers and/or photographicequipment use pigments based on dyes having absorption and as such theprinted colors can be insensitive to a change in the viewing angle.Therefore, the difference between an authentic document comprising colorshifting features and a fake one can be detected by tilting the documentto observe if there is a color shift. Some color shifting features thatare available are opaque and exhibit a color shift for reflection mode.Additionally, counterfeiters have developed sophisticated methods thatcompromise the effectiveness the existing reflective color shiftingfeatures as counterfeit protection. Thus, with respect to securitydevices, a new security feature that is difficult to counterfeit and canbe readily incorporated into an item such as a banknote is desirable.

SUMMARY

This application discloses and contemplates a wide variety of structuresincluding some at least partially transmissive optical structures.Advantageously, variations of such at least partially transmissiveoptical structures can present a color shift in both reflection mode andtransmission mode with respect to viewing angle. Also, variations ofsuch at least partially transmissive optical structures can beintegrated with documents (e.g., a banknote), packaging as well aspotential other items to, for example, enhance security and/or preventcounterfeiting. Although such features described herein can be used insecurity applications such as reducing the incidence of counterfeiting,alternatively or in addition, such feature could be used for providingan aesthetic effect or for other reasons.

This application contemplates documents, products, and packaging withfeatures (e.g., security features) that provides an optical effect ofchanging color with angle of observation in both reflection andtransmission. The color shift with respect to viewing angle in bothreflection and transmission can be achieved by incorporating the atleast partially transmissive optical structures in the document,product, packaging etc., as a security feature. The at least partiallytransmissive optical structures can be a dichroic structure. The atleast partially transmissive optical structures can be in the form of athin film coating on a flexible support or base layer such as a sheet,web or carrier. In some embodiments, the at least partially transmissiveoptical structures comprise a pigment. In some cases, an assembly ofparticles comprising the at least partially transmissive opticalstructures can be included in a medium and form, for example an ink. Theoptical effect from the assembly of particles can provide a color shiftin reflection and transmission. The color in transmission may be thecompliment color of the color perceived in reflection mode. In some suchembodiments, each particle can comprise the same structure or similarstructures.

Some implementations of the at least partially transmissive opticalstructures contemplated herein can comprise at least two metal layersthat sandwich at least one transparent layer between the at least twometal layers. The at least one transparent layer sandwiched between theat least two metal layers can have a refractive index that is greaterthan, less than or equal to 1.65. The at least partially transmissiveoptical structures contemplated herein can further comprise transparentlayers on the other side of the at least two metal layers. Thetransparent layers on the side of the at least two metal layers oppositethe side facing the sandwiched at least one transparent layer can have arefractive index greater than or equal to 1.65. The at least two metallayers can comprise metals that have a ratio of their real (n) andimaginary (k) refractive index less than 1.0. Accordingly, the metals ofthe at least two metal layers can have the ratio n/k<1. Without any lossof generality, the real part n is the refractive index and indicates thephase velocity, while the imaginary part k is called the extinctioncoefficient and can relate to absorption. The at least two metal layerscan comprise silver, silver alloys, aluminum, gold, as well as othermetals or materials or combination thereof.

Various optical structures contemplated in this application can providecolor shift when viewed in reflection and transmission mode as afunction of viewing angle. Hence these structures can be incorporated assecurity features for documents such as banknotes or other documents toverify authenticity of the documents. Structures contemplated in thisapplication can be configured to be used as a security thread, as alaminate, as a hot stamp, as a window patch or as pigment. The laminatecomprising a substrate (e.g., PET), the dichroic thin film and theprotective UV cured resin can be adhered as a unit to the banknote withan adhesive. Structures contemplated in this application can beconfigured to be used in a printing ink. Non-shifting transparent dyesor pigments can be incorporated with the optical structures contemplatedin this application to obtain new colors when viewed in reflection andtransmission mode. It is further contemplated that the two or more atleast partially transmissive optical structures can be disposed overeach other (e.g., printed or laminated over each other) to produceunique color effects. The at least partially transmissive opticalstructures contemplated herein can be configured or arranged to form,include or otherwise display text, symbols, numbers or figures thatappear and/or disappear in reflection or transmission as the viewingangle of the security device is changed. In other configurations, thefigures, images, numbers, pictures or symbols can be viewed atsubstantially all angles in transmission. For example, if the figures,images, numbers, pictures or symbols are printed in black, then they canbe viewed at substantially all angles in transmission. In some cases,for example text, numbers, pictures or symbols can be underprintedand/or overprinted under and/or over the at least partially transmissiveoptical structures using existing printing technologies.

The at least partially transmissive optical structures can be includedin or on or configured as a film, a foil, a coating, a pigment or anink. When configured as a pigment, in some implementations, the pigmentcan be encapsulated with a protective layer. The protective layer cancomprise SiO₂. The protective layer can comprise a solution preparedusing a sol-gel technology such as, for example, acid or based catalyzedtetraethylorthosilicate (TEOS) reactions for increased durability. Insome cases, the protective layer can further comprise silica sphereshaving same or different sizes. A silane coupling agent can be bondedwith the protective layer comprising silica (SiO₂). The silane couplingagent can be bonded to a resin, ink or paint vehicle. The resin, ink orpaint vehicle can comprise a material, such as, for example, acrylicmelamine, urethanes, polyesters, vinyl resins, acrylates, methacrylate,ABS resins, epoxies, styrenes and formulations based on alkyd resins andcombinations or mixtures thereof. In some implementations, the at leastpartially transmissive optical structures can be encapsulated, forexample, with an encapsulating layer having a refractive index thatmatches or closely matches the refractive index of the article to whichit is applied. In certain implementations, the encapsulating layer cancomprise a rough surface so that particles will not tend to sticktogether or stick to print rollers. The encapsulating layer can comprisea UV curing polymer.

These and other aspects of the at least partially transmissive opticalstructures will be apparent from their accompanying drawings and thisspecification.

The at least partially transmissive optical structures disclosed hereincan be used for security features included in documents, products,packages, etc., in particular, as security threads in bank notes or as alaminated strip, or as a patch or as a window. Other items such aspassports, ID cards, chip cards, credit cards, stock certificates orother investment securities, vouchers, admission tickets as well ascommercial packages that protect items of value such as CD's, medicinaldrugs, car and aircraft parts, etc. may also be protected againstcounterfeiting using the concepts and embodiments described herein.Furthermore, the at least partially transmissive optical structuresdisclosed herein can also be used for non-security applications.

Although some of the optical structures discussed herein can providecolor shift with viewing angle, optical structures that do not exhibitcolor shift with change in viewing angle or produce very little colorshift with change in viewing angle are also contemplated.

The systems, methods and devices disclosed herein each have severalinnovative aspects, no single one of which is solely responsible for thedesirable attributes disclosed herein. A variety of example systems andmethods are provided below.

Example 1

An optical structure comprising:

a first transparent dielectric layer having a refractive index greaterthan or equal to 1.65;

a first metal layer disposed over the first transparent dielectriclayer, the first metal layer having a first refractive index, wherein aratio of the real part (n) of the first refractive index to theimaginary part (k) of the first refractive index (k) is greater than orequal to 0.01 and less than or equal to 0.5;

a second transparent dielectric layer disposed over the first metallayer;

a second metal layer disposed over the second transparent dielectriclayer, the second metal layer having a second refractive index, whereina ratio of the real part (n) of the second refractive index to theimaginary part (k) of the second refractive index is greater than orequal to 0.01 and less than or equal to 0.5; and

a third transparent dielectric layer disposed over the second metallayer, the third transparent dielectric layer having a refractive indexgreater than or equal to 1.65.

Example 2

The optical structure of Example 1, wherein the second transparentdielectric layer has a refractive index less than 1.65.

Example 3

The optical structure of any of Examples 1-2, wherein the secondtransparent dielectric layer has a refractive index greater than orequal to 1.65.

Example 4

The optical structure of any of Examples 1-3, having a transmission peakcomprising:

a maximum transmittance greater than 50%; and

a spectral bandwidth defined by a full width of the transmission peak at50% of the maximum transmittance,

wherein the maximum transmittance is at least 50%, and

wherein the spectral bandwidth of the transmission peak is greater than2 nm.

Example 5

The optical structure of Example 4, wherein the spectral bandwidth ofthe transmission peak is greater than or equal to about 10 nm and lessthan or equal to about 200 nm.

Example 6

The optical structure of any of Examples 4-5, wherein the maximumtransmittance is at a wavelength between about 400 nm and about 700 nm.

Example 7

The optical structure of any of Examples 4-6, further comprising areflection peak comprising:

a maximum reflectance; and

a spectral bandwidth defined by a full width of the reflection peak at50% of the maximum reflectance,

wherein the maximum reflectance is at least 50%, and

wherein the spectral bandwidth of the reflection peak is greater than 2nm.

Example 8

The optical structure of Example 7, wherein the spectral bandwidth ofthe reflection peak is greater than or equal to about 10 nm and lessthan or equal to about 200 nm.

Example 9

The optical structure of any of Examples 7-8, wherein the maximumreflectance is at a wavelength between about 400 nm and about 700 nm.

Example 10

The optical structure of any of Examples 7-9, wherein the maximumtransmittance is at a first wavelength, and wherein the maximumreflectance is at a second wavelength different from the firstwavelength.

Example 11

The optical structure of any of Examples 1-10, configured to display afirst color when viewed by an average human eye along a direction normalto a surface of the optical structure in reflection mode and a secondcolor different from the first color when viewed by an average human eyealong a direction normal to a surface of the optical structure intransmission mode.

Example 12

The optical structure of Example 11, wherein the first color shifts to athird color when viewed by an average human eye along a direction at anangle away from the normal to the surface of the optical structure inreflection mode.

Example 13

The optical structure of any of Examples 11-12, wherein the second colorshifts to a fourth color when viewed by an average human eye along adirection at an angle away from the normal to the surface of the opticalstructure in transmission mode.

Example 14

The optical structure of any of Examples 1-13, wherein the first or thesecond metal layer has a thickness greater than or equal to about 5 nmand less than or equal to about 35 nm.

Example 15

The optical structure of any of Examples 1-14, wherein the secondtransparent dielectric layer has a thickness greater than or equal toabout 100 nm and less than or equal to about 2 microns.

Example 16

The optical structure of any of Examples 1-15, wherein first or thethird transparent dielectric layer has a thickness greater than or equalto about 100 nm and less than or equal to about 500 nm.

Example 17

The optical structure of any of Examples 1-16, further comprising anencapsulating layer comprising silica.

Example 18

The optical structure of Example 17, wherein the silica is bonded to asilane coupling agent.

Example 19

The optical structure of Example 18, wherein the silane coupling agentis configured to bind to an ink or paint medium.

Example 20

The optical structure of any of Examples 1-19, wherein the first or thesecond metal layer comprises at least one of aluminum, silver, gold,silver alloy, or gold alloy.

Example 21

The optical structure of any of Examples 1-20, wherein the secondtransparent dielectric layer comprises a material having a refractiveindex less than 1.65, greater than 1.65 or equal to 1.65.

Example 22

The optical structure of any of Examples 1-21, wherein the secondtransparent dielectric layer comprises at least one of SiO2, MgF2 or apolymer.

Example 23

The optical structure of any of Examples 1-22, wherein the first or thethird transparent dielectric layer comprises at least one of zinc oxide(ZnO), zinc sulfide (ZnS), zirconium dioxide (ZrO2), titanium dioxide(TiO2), tantalum pentoxide (Ta2O5), ceric oxide (CeO2), ytterium oxide(Y2O3), indium oxide (In2O3), tin oxide (SnO2), indium tin oxide (ITO),tungsten trioxide (WO3), or combinations thereof.

Example 24

The optical structure of any of Examples 1-23, wherein the first or thesecond metal layer has a thickness greater than or equal to about 5 nm,or less than or equal to about 35 nm.

Example 25

The optical structure of any of Examples 1-24, wherein the secondtransparent dielectric layer has a thickness greater than or equal toabout 100 nm, or less than or equal to about 700 nm.

Example 26

The optical structure of any of Examples 1-25, wherein the first or thethird transparent dielectric layer has a thickness greater than or equalto about 100 nm, or less than or equal to about 500 nm.

Example 27

The optical structure of any of Examples 1-26, configured as a pigment,a paint or an ink.

Example 28

The optical structure of any of Examples 1-27, further comprising a baselayer configured to support the first dielectric layer, wherein theoptical structure is configured as film.

Example 29

The optical structure of Example 28, wherein the base layer is flexible.

Example 30

The optical structure of any of Examples 28-29, wherein the base layercomprises a polymer.

Example 31

The optical structure of any of Examples 28-30, wherein the film issurrounded by a protective barrier.

Example 32

The optical structure of Example 31, wherein the protective barriercomprises a UV curable resin.

Example 33

The optical structure of any of Examples 1-32, further comprising anencapsulating layer, wherein the optical structure is configured as apigment, a paint or an ink.

Example 34

The optical structure of Example 33, wherein the encapsulating layercomprises silicon dioxide (SiO2).

Example 35

The optical structure of any of Examples 33-34, further comprising aplurality of silica spheres embedded in the encapsulating layer.

Example 36

The optical structure of Example 35, wherein some of the plurality ofsilica spheres have a size different from a size of some other of theplurality of silica spheres.

Example 37

The optical structure of any of Examples 33-36, wherein theencapsulating layer is chemically attached to a silane coupling agent,the silane coupling agent comprising a reactive group that is configuredto chemically bond with an ink or a paint medium.

Example 38

The optical structure of Example 37, wherein the ink or the paint mediumcomprises a material selected from the group consisting of acrylicmelamine, urethanes, polyesters, vinyl resins, acrylates, methacrylate,ABS resins, epoxies, styrenes and formulations based on alkyd resins andmixtures thereof.

Example 39

The optical structure of any of Examples 37-38, wherein the ink or thepaint medium comprises a resin or a polymer.

Example 40

A banknote or a document comprising the optical structure of any ofExamples 1-39.

Example 41

The banknote or document of Example 40, wherein the optical structure isconfigured as laminate that is attached to the banknote or document.

Example 42

The banknote or document of Example 40, wherein the optical structure isconfigured as a security thread that is inserted in the banknote ordocument.

Example 43

The banknote or document of Example 40, wherein the optical structure isconfigured as a label that is attached to the banknote or document.

Example 44

The banknote or document of Example 40, further comprising a window,wherein the optical structure is incorporated in the window.

Example 45

A document having a security feature comprising:

a main body of the document; and

an optical structure comprising:

a first transparent dielectric layer having a refractive index greaterthan or equal to 1.65;

a first metal layer disposed over the first transparent dielectriclayer, the first metal layer having a first refractive index, wherein aratio of the real part (n) of the first refractive index to theimaginary part (k) of the first refractive index (k) is greater than orequal to 0.01 and less than or equal to 0.5;

a second transparent dielectric layer disposed over the first metallayer;

a second metal layer disposed over the second transparent dielectriclayer, the second metal layer having a second refractive index, whereina ratio of the real part (n) of the second refractive index to theimaginary part (k) of the second refractive index is greater than orequal to 0.01 and less than or equal to 0.5; and

a third transparent dielectric layer having a refractive index greaterthan or equal to 1.65 disposed over the second metal layer,

wherein the optical structure is configured to display a first color inreflection mode and display a second color different from the firstcolor in transmission mode.

Example 46

The security document of Example 45, further comprising a second opticalstructure comprising:

a fourth transparent dielectric layer having a refractive index greaterthan or equal to 1.65;

a third metal layer disposed over the fourth transparent dielectriclayer, the third metal layer having a third refractive index, wherein aratio of the real part (n) of the third refractive index to theimaginary part (k) of the third refractive index (k) is greater than orequal to 0.01 and less than or equal to 0.5;

a fifth transparent dielectric layer disposed over the third metallayer;

a fourth metal layer disposed over the fifth transparent dielectriclayer, the fourth metal layer having a fourth refractive index, whereina ratio of the real part (n) of the fourth refractive index to theimaginary part (k) of the fourth refractive index is greater than orequal to 0.005 and less than or equal to 0.5; and

a sixth transparent dielectric layer having a refractive index greaterthan or equal to 1.65 disposed over the fourth metal layer,

wherein the second optical structure is configured to display a thirdcolor in reflection mode different from the first and the second colorand display a fourth color different from the first, second and thethird color in transmission mode.

Example 47

The security document of Example 46, wherein the optical structure orthe second optical structure is configured as a film attached to themain body of the document.

Example 48

The security document of any of Examples 46-47, wherein the opticalstructure or the second optical structure is configured as a threadinserted into the main body of the document.

Example 49

The security document of any of Examples 46-48, wherein the opticalstructure or the second optical structure is configured as a laminatedisposed over the main body of the document.

Example 50

The security document of any of Examples 46-49, wherein the opticalstructure or the second optical structure is configured as an ink, adye, or a paint contacting the main body of the document.

Example 51

The security document of any of Examples 46-50, further comprising afirst window comprising the optical structure and a second windowcomprising the second optical structure.

Example 52

The security document of any of Examples 46-51, wherein the opticalstructure is configured as a dichroic ink, a dichroic pigment or adichroic paint that is configured to produce a first color at a firstviewing angle and a second color at a second viewing angle.

Example 53

The security document of any of Examples 46-52, wherein the document isprinted with the dichroic ink, the dichroic pigment or the dichroicpaint.

Example 54

The security document of Example 53, wherein the dichroic ink, thedichroic pigment or the dichroic paint is disposed over, under or mixedwith a non-dichroic ink, pigment, or paint that is configured to producethe first color at the first and the second viewing angles.

Example 55

The security document of Example 54, wherein the non-dichroic, inkpigment or paint forms a text, an image, a number or a symbol.

Example 56

The security document of Example 55, wherein the text, the image, thenumber or the symbol is invisible at the first viewing angle and visibleat the second viewing angle.

Example 57

A method of manufacturing a security feature configured to produce afirst color in reflection mode and a second color in transmission mode,the method comprising:

providing a base layer; and

disposing an optical structure on the base layer, the optical structurecomprising:

-   -   a first transparent dielectric layer on the base layer, the        first transparent dielectric layer having a refractive index        greater than or equal to 1.65;    -   a first metal layer disposed over the first transparent        dielectric layer, the first metal layer having a first        refractive index, wherein a ratio of the real part (n) of the        first refractive index to the imaginary part (k) of the first        refractive index (k) is greater than or equal to 0.01 and less        than or equal to 0.5;    -   a second transparent dielectric layer disposed over the first        metal layer;    -   a second metal layer disposed over the second transparent        dielectric layer, the second metal layer having a second        refractive index, wherein a ratio of the real part (n) of the        second refractive index to the imaginary part (k) of the second        refractive index is greater than or equal to 0.01 and less than        or equal to 0.5; and    -   a third transparent dielectric layer disposed over the second        metal layer, the third dielectric layer having a refractive        index greater than or equal to 1.65.

Example 58

The method of Example 57, wherein disposing the optical structure on thebase layer comprises:

coating the first transparent dielectric layer on the base layer;

depositing the first metal layer on the first transparent dielectriclayer;

disposing the second transparent dielectric layer on the first metallayer;

depositing the second metal layer on the second transparent dielectriclayer; and

disposing the third transparent dielectric layer on the second metallayer.

Example 59

The method of any of Examples 57-58, further comprising:

cutting a strip of the base layer with the optical structure; and

coating the strip with a UV curable polymer to obtain a security thread.

Example 60

The method of any of Examples 57-58, further comprising:

removing the optical structure from the base layer;

fragmenting optical structure into platelets having an area that isbetween five times and about ten times the thickness of the opticalstructure;

encapsulating the platelet in an encapsulation layer comprising aplurality of silica spheres;

attaching a silane coupling agent to the encapsulating layer; and

mixing the platelets with an ink or a paint medium to obtain a dichroicink or paint.

Example 61

The method of any of Examples 57-60, wherein the base layer is flexible.

Example 62

The method of any of Examples 57-61, wherein the base layer compriseweb.

Example 63

An optical structure comprising:

a substrate;

a first optical structure over the substrate; and

a second optical structure over the substrate, the first opticalstructure and the second optical structure at least partiallyoverlapping,

wherein the each of the first and the second optical structurecomprises:

a first transparent dielectric layer having a refractive index greaterthan or equal to 1.65;

a first metal layer disposed over the first transparent dielectriclayer, the first metal layer having a first refractive index, wherein aratio of the real part (n) of the first refractive index to theimaginary part (k) of the first refractive index (k) is greater than orequal to 0.01 and less than or equal to 0.5;

a second transparent dielectric layer disposed over the first metallayer;

a second metal layer disposed over the second transparent dielectriclayer, the second metal layer having a second refractive index, whereina ratio of the real part (n) of the second refractive index to theimaginary part (k) of the second refractive index is greater than orequal to 0.01 and less than or equal to 0.5; and

a third transparent dielectric layer disposed over the second metallayer, the third transparent dielectric layer having a refractive indexgreater than or equal to 1.65,

wherein a thickness of the various layers of the first optical structureis configured to reflect a first color and transmit a second colordifferent from the first color, and

wherein a thickness of the various layers of the second opticalstructure is configured to reflect a third color different from thefirst color and transmit a fourth color different from the first, thesecond or the third color.

Example 64

The optical structure of Example 63, wherein the first and the secondoptical structures are completely overlapping.

Example 65

The optical structure of any of Examples 63-64, wherein the first andthe second optical structures are configured as films.

Example 66

The optical structure of any of Examples 63-65, wherein the first andthe second optical structures are configured as pigments.

Example 67

The optical structure of any of Examples 63-66, wherein the first andthe second optical structures are configured as laminates.

Example 68

The optical structure of any of Examples 63-67, wherein the first andthe second optical structures are configured as security threads.

Example 69

A document having a security feature comprising:

a main body of the document; and

a pigment disposed on the main body, the pigment comprising:

an optical structure comprising:

a first metal layer disposed over the first transparent dielectriclayer, the first metal layer having a first refractive index, wherein aratio of the real part (n) of the first refractive index to theimaginary part (k) of the first refractive index (k) is greater than orequal to 0.01 and less than or equal to 0.5;

a transparent dielectric layer disposed over the first metal layer; and

a second metal layer disposed over the transparent dielectric layer, thesecond metal layer having a second refractive index, wherein a ratio ofthe real part (n) of the second refractive index to the imaginary part(k) of the second refractive index is greater than or equal to 0.01 andless than or equal to 0.5; and

an encapsulation layer encapsulating the optical structure.

Example 70

The document of Example 69, wherein the encapsulation layer comprisessilica.

Example 71

The document of any of Examples 69-70, wherein the pigment produces afirst color at a first viewing angle and a second color different fromthe first color at a second viewing angle.

Example 72

The document of any of Examples 69-71, wherein the pigment comprises aresin configured to chemically attach to the encapsulation layer.

Example 73

The document of any of Examples 69-72, wherein the optical structure hasa thickness, and wherein a length or a width of the optical structure isat least 5 times the thickness.

Example 74

An optical structure comprising:

a dielectric region having an outer surface enclosing a volume of adielectric material; and

a partially optically transmissive metal layer surrounding the outersurface of the dielectric region,

wherein a thickness of the optical structure has a value between about100 nm and about 2 micron,

wherein a lateral dimension of the optical structure is between about 1micron and about 20 micron, and

wherein the optical structure is configured to display a first color ina reflection mode and display a second color different from the firstcolor in transmission mode.

Example 75

The optical structure of Example 74, further comprising a seconddielectric region comprising one or more dielectric materials having arefractive index greater than about 1.65, the second dielectric regionsurrounding the partially optically transmissive metal layer.

Example 76

The optical structure of any of Examples 74-75, wherein the partiallyoptically transmissive metal layer covers at least 80% of the outersurface of the dielectric region.

Example 77

The optical structure of any of Examples 74-76, wherein the partiallyoptically transmissive metal layer covers at least 90% of the outersurface of the dielectric region.

Example 78

The optical structure of any of Examples 74-77, wherein the partiallyoptically transmissive metal layer covers 100% of the outer surface ofthe dielectric region.

Example 79

The optical structure of any of Examples 74-78, wherein the dielectricregion is spherical, ellipsoidal or round.

Example 80

The optical structure of any of Examples 74-79, wherein the dielectricregion is a cube or a rectangular cuboid.

Example 81

The optical structure of any of Examples 74-80, wherein the dielectricregion comprises a particle.

Example 82

The optical structure of any of Examples 74-81, wherein the partiallyoptically transmissive metal layer comprises silver.

Example 83

The optical structure of any of Examples 74-82, wherein the partiallyoptically transmissive metal layer has a thickness between about 3 nmand about 40 nm.

Example 84

The optical structure of any of Examples 74-83, wherein the dielectricregion comprises silicon dioxide or titanium dioxide.

Example 85

The optical structure of any of Examples 75-84, wherein the seconddielectric layer comprises a material having a refractive index greaterthan about 1.65.

Example 86

The optical structure of any of Examples 75-85, wherein the seconddielectric layer comprises titanium dioxide.

Example 87

The optical structure of any of Examples 75-86, wherein the seconddielectric layer covers at least 80% of the outer surface of thepartially optically transmissive metal layer.

Example 88

The optical structure of any of Examples 75-87, wherein the seconddielectric layer covers at least 90% of the outer surface of thepartially optically transmissive metal layer.

Example 89

The optical structure of any of Examples 75-88, wherein the seconddielectric layer covers at least 95% of the outer surface of thepartially optically transmissive metal layer.

Example 90

The optical structure of any of Examples 75-89, wherein the seconddielectric layer covers 100% of the outer surface of the partiallyoptically transmissive metal layer.

Example 91

The optical structure of any of Examples 74-90, wherein said dielectricregion comprises SiO₂.

Example 92

The optical structure of any of Examples 74-91, wherein said dielectricregion comprises TiO₂.

Example 93

The optical structure of any of Examples 74-92, wherein said dielectricregion comprises borosilicate with a high refractive index metal oxidelayer thereon.

Example 94

The optical structure of any of Examples 74-93, wherein said dielectricregion comprises borosilicate with TiO₂ thereon.

Example 95

The optical structure of any of Examples 74-94, wherein said dielectricregion comprises borosilicate with SiO₂ thereon.

Example 96

The optical structure of any of Examples 74-95, included in a securitythread or security ink.

Example 97

The optical structure of any of Examples 74-95, included in a film, athread, a foil, or a laminate.

Example 98

The optical structure of any of Examples 74-95, included in a flexiblefilm having a flexible base.

Example 99

The optical structure of any of Examples 74-95, included in a pigment, apaint or an ink.

Example 100

A security document comprising the optical structure of any of Examples74-99.

Example 101

A security document comprising the optical structure of any of Claims74-100, wherein the first color and second color are complementarycolors.

Example 102

A method of manufacturing a dichroic ink or paint configured to producea first color in reflection mode and a second color in transmissionmode, the method comprising:

providing a base layer; and

disposing an optical structure on the base layer, the optical structurecomprising:

a first metal layer disposed on the base layer, the first metal layerhaving a first refractive index, wherein a ratio of the real part (n) ofthe first refractive index to the imaginary part (k) of the firstrefractive index (k) is greater than or equal to 0/01 and less than orequal to 0.5;

a first transparent dielectric layer disposed over the first metallayer; and

a second metal layer disposed over the first transparent dielectriclayer, the second metal layer having a second refractive index, whereina ratio of the real part (n) of the second refractive index to theimaginary part (k) of the second refractive index is greater than orequal to 0.01 and less than or equal to 0.5.

Example 103

The method of Example 102, further comprising:

removing the optical structure from the base layer;

fragmenting optical structure into platelets having an area that isbetween five times and about ten times the thickness of the opticalstructure; and

dispersing the platelets in an ink medium or a paint medium to obtain adichroic ink or paint.

Example 104

The method of Example 103, further comprising encapsulating anindividual platelet in an encapsulation layer comprising a plurality ofsilica spheres.

Example 105

The method of Example 104, further comprising attaching a silanecoupling agent to the encapsulating layer.

Example 106

The method of any of Examples 102-105, wherein the optical structurefurther comprises:

a second transparent dielectric layer between the base layer and thefirst metal layer, the second transparent dielectric layer having arefractive index greater than or equal to 1.65; anda third transparent dielectric layer disposed over the second metallayer, the third dielectric layer having a refractive index greater thanor equal to 1.65.

Example 107

A dichroic ink or paint configured to produce a first color inreflection mode and a second color in transmission mode, the dichroicink or paint comprising:

a base layer; and

an optical structure on the base layer, the optical structurecomprising:

a first metal layer disposed on the base layer, the first metal layerhaving a first refractive index, wherein a ratio of the real part (n) ofthe first refractive index to the imaginary part (k) of the firstrefractive index (k) is greater than or equal to 0.01 and less than orequal to 0.5;

a first transparent dielectric layer disposed over the first metallayer; and

a second metal layer disposed over the first transparent dielectriclayer, the second metal layer having a second refractive index, whereina ratio of the real part (n) of the second refractive index to theimaginary part (k) of the second refractive index is greater than orequal to 0.01 and less than or equal to 0.5.

Example 108

The dichroic ink or paint of Claim 107, wherein the optical structurefurther comprises:

a second transparent dielectric layer between the base layer and thefirst metal layer, the second transparent dielectric layer having arefractive index greater than or equal to 1.65; and

a third transparent dielectric layer disposed over the second metallayer, the third dielectric layer having a refractive index greater thanor equal to 1.65.

Example 109

The dichroic ink or paint of any of Claims 107-108, further comprisingan ink medium or a paint medium comprising the optical structure,wherein the optical structure has a thickness between 100 nm and 2micron, and wherein a lateral dimension of the optical structure isbetween 1 micron and 20 micron.

Example 110

An optical structure comprising:

a dielectric region having an outer surface enclosing a volume of adielectric material; and

a partially optically transmissive metal layer surrounding the outersurface of the dielectric region,

wherein a thickness of the optical structure has a value between about100 nm and about 2 micron,

wherein a lateral dimension of the optical structure is between about100 nm and about 20 micron, and

wherein the optical structure is configured to display a first color ina reflection mode and display a second color different from the firstcolor in transmission mode.

Example 111

The optical structure of Example 110, further comprising a seconddielectric region comprising one or more dielectric materials having arefractive index greater than about 1.65, the second dielectric regionsurrounding the partially optically transmissive metal layer.

Example 112

The optical structure of any of Examples 110-111, wherein the partiallyoptically transmissive metal layer covers at least 80% of the outersurface of the dielectric region.

Example 113

The optical structure of any of Examples 110-112, wherein the partiallyoptically transmissive metal layer covers at least 90% of the outersurface of the dielectric region.

Example 114

The optical structure of any of Examples 110-113, wherein the partiallyoptically transmissive metal layer covers 100% of the outer surface ofthe dielectric region.

Example 115

The optical structure of any of Examples 110-114, wherein the dielectricregion is spherical, ellipsoidal or round.

Example 116

The optical structure of any of Examples 110-115, wherein the dielectricregion is a cube or a rectangular cuboid.

Example 117

The optical structure of any of Examples 110-116, wherein the dielectricregion comprises a particle.

Example 118

The optical structure of any of Examples 110-117, wherein the partiallyoptically transmissive metal layer comprises silver.

Example 119

The optical structure of any of Examples 110-118, wherein the partiallyoptically transmissive metal layer has a thickness between about 3 nmand about 40 nm.

Example 120

The optical structure of any of Examples 110-119, wherein the dielectricregion comprises silicon dioxide or titanium dioxide.

Example 121

The optical structure of any of Examples 111-120, wherein the seconddielectric layer comprises a material having a refractive index greaterthan about 1.65.

Example 122

The optical structure of any of Examples 111-121, wherein the seconddielectric layer comprises titanium dioxide.

Example 123

The optical structure of any of Examples 111-122, wherein the seconddielectric layer covers at least 80% of the outer surface of thepartially optically transmissive metal layer.

Example 124

The optical structure of any of Examples 111-123, wherein the seconddielectric layer covers at least 90% of the outer surface of thepartially optically transmissive metal layer.

Example 125

The optical structure of any of Examples 111-124, wherein the seconddielectric layer covers at least 95% of the outer surface of thepartially optically transmissive metal layer.

Example 126

The optical structure of any of Examples 111-125, wherein the seconddielectric layer covers 100% of the outer surface of the partiallyoptically transmissive metal layer.

Example 127

The optical structure of any of Examples 110-126, wherein saiddielectric region comprises SiO₂.

Example 128

The optical structure of any of Examples 110-127, wherein saiddielectric region comprises TiO₂.

Example 129

The optical structure of any of Examples 110-128, wherein saiddielectric region comprises borosilicate with a high refractive indexmetal oxide layer thereon.

Example 130

The optical structure of any of Examples 110-129, wherein saiddielectric region comprises borosilicate with TiO₂ thereon.

Example 131

The optical structure of any of Examples 110-130, wherein saiddielectric region comprises borosilicate with SiO₂ thereon.

Example 132

The optical structure of any of Examples 110-131, included in a securitythread or security ink.

Example 133

The optical structure of any of Examples 110-132, included in a thread,a foil, or a laminate.

Example 134

The optical structure of any of Examples 110-133, included in a flexiblefilm having a flexible base.

Example 135

The optical structure of any of Examples 110-134, included in a pigment,a paint or an ink.

Example 136

A security document comprising the optical structure of any of Examples110-135.

Example 137

A security document comprising the optical structure of any of claims110-136, wherein the first color and second color are complementarycolors.

Example 138

The optical structure of any of Examples 1-26, configured as a foil, athread or a laminate.

Example 139

The optical structure of any of Examples 110-112, wherein the partiallyoptically transmissive metal layer covers at least 95% of the outersurface of the dielectric region.

Example 140

The optical structure of any of Examples 74-76, wherein the partiallyoptically transmissive metal layer covers at least 95% of the outersurface of the dielectric region.

Example 141

The security document of Example 55, wherein the text, the image, thenumber or the symbol is invisible at the second viewing angle andvisible at the first viewing angle.

Example 142

The method of Example 58, wherein disposing the second transparentdielectric layer on the first metal layer comprises depositing thesecond transparent dielectric layer on the first metal layer.

Example 143

The method of Example 58, wherein disposing the third transparentdielectric layer on the second metal layer comprises depositing thethird transparent dielectric layer on the second metal layer.

Example 144

The method of Examples 57 or 58, further comprising:

removing the optical structure from the base layer;

fragmenting optical structure into platelets having an area that isbetween five times and about ten times the thickness of the opticalstructure;

attaching a silane coupling agent to the optical structure; and

mixing the platelets with an ink or a paint medium to obtain a dichroicink or paint.

Example 145

The method of Example 144, further comprising:

encapsulating the platelet in an encapsulation layer; and

attaching the silane coupling agent to the encapsulation layer.

Example 146

The method of Example 58, further comprising depositing the first metallayer on the first transparent dielectric layer using an electrolessmethod.

Example 147

The method of Example 58, further comprising depositing the second metallayer on the second transparent dielectric layer using an electrolessmethod.

Example 148

A pigment comprising:

an optical structure comprising:

-   -   a first metal layer disposed over the first transparent        dielectric layer, the first metal layer having a first        refractive index, wherein a ratio of the real part (n) of the        first refractive index to the imaginary part (k) of the first        refractive index (k) is greater than or equal to 0.01 and less        than or equal to 0.5;    -   a transparent dielectric layer disposed over the first metal        layer; and    -   a second metal layer disposed over the transparent dielectric        layer, the second metal layer having a second refractive index,        wherein a ratio of the real part (n) of the second refractive        index to the imaginary part (k) of the second refractive index        is greater than or equal to 0.01 and less than or equal to 0.5.

Example 149

The pigment of Example 148, further comprising an encapsulation layerencapsulating the optical structure.

Example 150

The pigment of Example 149, wherein the encapsulation layer comprisessilica.

Example 151

The pigment of any of Examples 148-150, further comprising a resinconfigured to chemically attach to the encapsulation layer.

Example 152

The pigment of any of Examples 148-151, configured to produce a firstcolor at a first viewing angle and a second color different from thefirst color at a second viewing angle.

Example 153

The pigment of any of Examples 148-152, wherein the optical structurehas a thickness, and wherein a length or a width of the opticalstructure is at least 5 times the thickness.

Example 154

A document comprising the pigment of any of Examples 148-153, thedocument comprising a main body and the pigment disposed on the mainbody.

Example 155

A packaging comprising the pigment of any of Examples 148-153, thepackaging comprising a main body and the pigment disposed on the mainbody.

Example 156

The optical structure of any of Examples 1-26, configured as a foil.

Example 157

The optical structure of any of Examples 1-26, configured as a thread.

Example 158

The optical structure of any of Examples 1-26, configured as a laminate.

Example 159

The optical structure of any of Examples 110-132, included in a thread.

Example 160

The optical structure of any of Examples 110-132, included in a foil.

Example 161

The optical structure of any of Examples 110-132, included in alaminate.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments will now be described in conjunction with thedrawings.

FIG. 1 schematically illustrates a side view of an optical structureconfigured to be used as a security feature.

FIGS. 2A-1 and 2A-2 schematically illustrate side views of opticalstructures configured to be used as a security feature in the form of aplatelet encapsulated with an encapsulating layer, comprising, forexample, a SiO₂ layer and silica spheres.

FIGS. 2B-1 and 2B-2 illustrates a plurality of platelets dispersed in apolymer which can comprise an ink or a paint medium.

FIG. 3 illustrates the silane coupling agent bonded to an exposedsurface of the encapsulation layer of a platelet. Another side of thesilane coupling agent can also bond to a medium such as a polymer inwhich the platelets are dispersed.

FIG. 4 is a schematic illustration showing propagation light incident onthe optical structure and the resultant nodes in field strength at themetal layers.

FIGS. 5A and 5B illustrate transmission and reflection spectra ofexamples of optical structures.

FIGS. 6A-6D and 7A-7D are a* b* plots showing the color travel or changein reflection and transmission respectively for four different exampleoptical structures.

FIGS. 8A and 8B respectively illustrate the transmittance andreflectance spectra for an example of the optical structure.

FIGS. 8C and 8D respectively illustrate the transmittance andreflectance spectrum for an example of the optical structure.

FIGS. 8E and 8F respectively illustrate the transmittance andreflectance spectrum for an example of the optical structure.

FIG. 8G illustrates the a*b* values in the CIELa*b* color space for anexample of the optical structure for different viewing angles between 0degrees and 40 degrees with respect to the normal to the surface of theexample of the optical structure in transmission mode.

FIG. 8H illustrates the a*b* values in the CIELa*b* color space for anexample of the optical structure for different viewing angles between 0degrees and 40 degrees with respect to the normal to the surface of theexample of the optical structure in reflection mode.

FIG. 9A schematically illustrates a cross-sectional view of anembodiment of an optical structure configured to be used as a securityfeature. FIG. 9B schematically illustrates a cross-sectional view ofanother embodiment of an optical structure configured to be used as asecurity feature.

FIG. 10 is a schematic illustration of a laminate structure comprisingan optical structure that is affixed to a banknote.

FIG. 11A shows a banknote with two windows, each window including adifferent optical structure. FIG. 11B shows a security device with twoat least partially overlapping windows, each window comprising adifferent optical structure.

FIGS. 12 and 13 illustrate examples of a security device comprising anoptical structure disposed under or over a text, symbol or number. Thetext, symbol or number becomes visible when the viewing angle ischanged.

DETAILED DESCRIPTION

To curtail counterfeiting, currency, documents (e.g., banknotes) as wellas other items such as products and packaging can be provided withsecurity features that can be inspected by the general public to verifyauthenticity. In many cases, it can be advantageous if the securityfeatures can be easily seen under a variety of light conditions andwithout the need for special lighting conditions. It can also bedesirable that the security features have distinct characteristics thatcan be easily identified by the public within a 1-10 second time frame.In addition, it is advantageous in general, if the security feature isnot susceptible to copying by electronic or photographic equipment, suchas, for example, printers, copiers, cameras, etc.

One example of a security feature employed in banknotes is thewatermark, which has a fairly high degree of awareness among the generalpublic. An example of a watermark can be an image comprising light anddark regions that can be easily seen by holding up the banknote to seethe watermark in light transmission. However, watermarks may besusceptible to be copied and thus are not very secure. Other examples ofsecurity features may use inks and motion type features that are notreadily seen under low light conditions (e.g., at low lit bars,restaurants, etc.), have poor image resolution, and/or have slow opticalmovement relative to the movement of the banknote. Accordingly, someexisting security features tend to be more complicated structures havingmore complex color changing effects. This approach, however, can bedisadvantageous when the complicated security devices are applied tobanknotes or currency, as these complicated security devices may confusean average person who is looking for a distinctive security feature.

Having a security features that has high contrast with respect to thebackground that can be easily identified by the general public under avariety of light conditions, including low light, can be advantageous.Accordingly, various security features disclosed can appears to have onecolor in reflection and another different color in transmission. Thesesecurity features can be incorporated in a banknote. A consumer,merchant, or a bank teller can holdup such a banknote to light toreadily verify the authenticity of the banknote. Additionally, in someimplementations, the security feature can be configured to exhibit colorshift and/or movement of identifiable features when the viewing angle isvaried to enhance security. These and other features are described infurther detail herein.

Accordingly, various security features contemplated herein can compriseoptical stacks and/or structures that are at least partially reflectiveand at least partially transmissive. The security features contemplatedherein can be configured as coatings, threads, laminates, foils, films,pigments and/or inks and incorporated with banknotes or other items. Theinnovative aspects described in this application also include systemsand methods of fabricating optical structures and/or stacks that are atleast partially reflective and at least partially transmissive. In someembodiments, such optical structures may be fabricated on support orbase layers or sheets such as webs (e.g., roll coated webs). Processesdescribed herein may also include removing the fabricated opticalstructures and/or stacks from a support or base layer (e.g., roll orsheet). The innovative aspects described in this application furtherincludes methods and systems for including the optical structures and/orstacks that are at least partially reflective and at least partiallytransmissive in pigment and inks having a desired amount of durabilityand mechanical strength to be further used in or on or incorporated intobanknotes and other security devices/documents.

FIG. 1 schematically illustrates an optical structure 10 comprising astack of layers that can be used as a security feature. The opticalstructure 10 comprises at least two metal layers 13 and 15. The at leasttwo metal layers 13 and 15 can comprise metals having a ratio of thereal part (n) of the refractive index to the imaginary part (k) of therefractive index (k) that is less than 1. For example, the at least twometal layers 13 and 15 can comprise metals that have an n/k valuebetween about 0.01 and about 0.6, between about 0.015 and about 0.6,between about 0.01 and about 0.5, between about 0.01 and about 0.2,between about 0.01 and about 0.1, or any value in a range or sub-rangedefined by these values. Accordingly, the at least two metal layers 13and 15 can comprise silver, silver alloys, gold, aluminum or copper andtheir respective alloys. Nickel (Ni) and Palladium (Pd) can be used insome implementations. In some cases, however, the at least two metallayers 13 and 15 do not comprise chromium, titanium, and/or tungsten orany metal having an n/k ratio greater than 0.6. In some cases, the metallayer 13 and 15 can have a thickness greater than or equal to about 3 nmand less than or equal to about 35 nm. For example, thickness of themetal layer 13 and 15 can be greater than or equal to about 10 nm andless than or equal to about 30 nm, greater than or equal to about 15 nmand less than or equal to about 27 nm, greater than or equal to about 20nm and less than or equal to about 25 nm, or any value in a range orsub-range defined by these values. The thickness of the metal layer 13can be equal to the thickness of the metal layer 15. Alternately, thethickness of the metal layer 13 can be greater than or less than thethickness of the metal layer 15.

A transparent dielectric layer 14 is sandwiched between the at least twometal layers 13 and 15. The dielectric layer 14 can have a refractiveindex greater than, less than or equal to 1.65. Materials with an indexgreater than or equal to 1.65 can be considered as high refractive indexmaterials for the purpose of this application and materials with anindex less than 1.65 can be considered as low index materials for thepurpose of this application. The transparent dielectric layer 14 cancomprise inorganic materials including but not limited to siliconedioxide (SiO₂), aluminum oxide (Al₂O₃), magnesium fluoride (MgF₂),cerium fluoride (CeF₃), lanthanum fluoride (LaF₃), zinc oxide (ZnO),zinc sulfide (ZnS), zirconium dioxide (ZrO₂), titanium dioxide (TiO₂),tantalum pentoxide (Ta₂O₅), ceric oxide (CeO₂), ytterium oxide (Y₂O₃),indium oxide (In₂O₃), tin oxide (SnO₂), indium tin oxide (ITO) andtungsten trioxide (WO₃) or combinations thereof. The transparentdielectric layer 14 can comprise polymers including but not limited toparylene, acrylates, and/or methacrylate. Without any loss ofgenerality, the transparent dielectric layer 14 can comprise a materialhaving an index of refraction greater than, less than, or equal to 1.65and an extinction coefficient between 0 and about 0.5 such that it haslow absorption of light in the visible spectral range.

The dielectric layer 14 can have a thickness that is greater than orequal to about 75 nm and less than or equal to about 2 micron. Forexample, the dielectric layer 14 can have a thickness that is greaterthan or equal to about 150 nm and less than or equal to about 650 nm,greater than or equal to about 200 nm and less than or equal to about600 nm, greater than or equal to about 250 nm and less than or equal toabout 550 nm, greater than or equal to about 300 nm and less than orequal to about 500 nm, greater than or equal to about 350 nm and lessthan or equal to about 450 nm, greater than or equal to about 700 nm andless than or equal to about 1 micron, greater than or equal to about 900nm and less than or equal to about 1.1 micron, greater than or equal toabout 1 micron and less than or equal to about 1.2 micron, greater thanor equal to about 1.2 micron and less than or equal to about 2.0 micronsor any value in a range/sub-range defined by these values. Withoutsubscribing to any particular theory, in various implementations, thethickness of the dielectric layer 14 can be approximately a quarterwavelength of light (e.g., visible light) incident thereon or an integermultiple of a quarter wavelength. In various implementations, thethickness of the dielectric layer 14 may be, for example, ¼, ¾, 5/4,7/4, 9/4, 10/4, etc. of the wavelength of visible light incident on thedielectric layer 14.

The optical structure 10 further comprises a transparent dielectriclayer 12 that is disposed on a side of the metal layer 13 that isopposite to the dielectric layer 14 and a transparent dielectric layer16 that is disposed on a side of the metal layer 15 that is opposite tothe dielectric layer 14. In some cases, layers 12 and 16 can comprisematerials having a refractive index greater than or equal to 1.65. Forexample, layers 12 and 16 can comprise ZrO₂, TiO₂, ZnS, ITO (indium tinoxide). CeO₂ or Ta₂O₃. Dielectric layers 12 and 16 can have a thicknessthat is greater than or equal to about 100 nm and less than or equal toabout 400 nm, greater than or equal to about 150 nm and less than orequal to about 350 nm, greater than or equal to about 200 nm and lessthan or equal to about 300 nm, or any value in a range/sub-range definedby these values. The thickness of the dielectric layer 12 can be equalto the thickness of the dielectric layer 16. Alternately, the thicknessof the dielectric layer 12 can be greater than or less than thethickness of the dielectric layer 16. The optical structure 10 can havea thickness that is less than or equal to about 2 microns.

Fabricating the optical structure 10 can include providing the layer ofdielectric material 12 (or the layer of dielectric material 16) anddepositing the metal layer 13 (or the metal layer 15) over the layer ofdielectric material 12 (or the layer of dielectric material 16). Themetal layer 13 (or the metal layer 15) can be deposited over the layerof dielectric material 12 (or the layer of dielectric material 16) usingan electroless method discussed in further detail below. The metal layer13 (or the metal layer 15) can be deposited as a continuous thin film,as small spheres, metallic clusters or island like structures. The otherdielectric layer 14 can be subsequently disposed over the metal layer 13(or the metal layer 15). The initial layer of dielectric material 12 (orthe layer of dielectric material 16) can be disposed and/or formed overa support. The support is also referred to herein as a base layer. Thesupport can comprise a carrier. The support can comprise a sheet such asa web. The support can comprise a substrate. The substrate can be acontinuous sheet of PET or other polymeric web structure. The supportcan comprise a non-woven fabric. Non-woven fabrics can be flat, poroussheets comprising fibers. In some implementations, the non-woven fabriccan be configured as a sheet or a web structure that is bonded togetherby entangling fiber or filaments mechanically, thermally, or chemically.In some implementations, the non-woven fabric can comprise perforatedfilms (e.g., plastic or molten plastic films). In some implementations,the non-woven fabric can comprise synthetic fibers such as polypropyleneor polyester or fiber glass.

The support can be coated with a release layer comprising a releaseagent. The release agent can be soluble in solvent or water. The releaselayer can be polyvinyl alcohol, which is water soluble or an acrylatewhich is soluble in a solvent. The release layer can comprise a coating,such as, for example, salt (NaCl) or cryolite (Na₃AlF₆) deposited byevaporation before the layers of the optical structure aredeposited/formed.

In some implementations of the support configured as a non-woven fabric,the non-woven fabric can be coated with a release layer. Suchimplementations can be dipped or immersed in a solvent or water thatacts as a release agent to dissolve or remove the release layer. Therelease agent (e.g., the solvent or water) is configured to penetratefrom a side of the non-woven fabric opposite the side on which theoptical structure is disposed to facilitate release of the opticalstructure instead of having to penetrate through the optical structure.The optical structure is recovered from the solvent or water afterdissolution of the release layer. In some manufacturing approaches, therecovered optical structure can then be processed into a pigment.

In one method of fabrication, the optical structure 10 can befabricated, for example, deposited or formed on a coated web, a coatedbase layer, a coated carrier or a coated substrate. The coating on theweb, the base layer, the substrate or the carrier can be configured as arelease layer to facilitate easy removal of the optical structure 10.

The optical structure 10 can be configured as a film or a foil bydisposing over a substrate or other support layer having a thickness,for example, greater than or equal to about 10 microns and less than orequal to about 25 microns. For example, a substrate or support layersuch as a polyester substrate or support layer can have a thicknessgreater than or equal to 12 microns and less than or equal to 22.5microns, greater than or equal to 15 microns and less than or equal toabout 20 microns. The substrate or support layer can comprise materials,such as, for example, polyester, polyethylene, polypropylene, orpolycarbonate. The support or support layer itself can be dissolvable.The support or support layer, for example, can also comprise polyvinylalcohol, which can be dissolved, for example, in water. Accordingly,instead of using a release layer on a insoluble support web, the supportweb itself may comprise soluble material. Accordingly, the support orsupport layer can be dissolved leaving the optical coating remaining.The optical structure 10 configured as a film or a foil can beencapsulated with a polymer, such as, for example a UV cured polymer.

The optical structure 10 can comprises additional layers. For example, athin protective layer may be disposed between the metal layer 13 and thedielectric layer 12 and/or between the metal layer 15 and the dielectriclayer 16. The protective layer can comprise materials, such as, forexample, NiCrO_(x), Si₃N₄, CeSnO₄ and ZnSnO₄. The protective layers canhave a thickness between about 3-5 nm. The protective layers canadvantageously increase the durability of the metal layers 13 and 15.

Instead of a film, the optical structure, 10, may be removed from thesubstrate, web, carrier, or support layer on which it is fabricated anddivided into platelets having a size that is suitable for a pigment orprinting ink. Platelets having a size that is suitable for a pigment orprinting ink can have an area, length, and/or width that is about 5-10times the thickness of the platelet, in some implementations.Accordingly, the platelets having a thickness of about 1 micron, and/orcan have a width and/or a length that is between approximately 5 micronand about 50 microns. For example, the width and/or a length can begreater than or equal to about 5 micron and less than or equal to about15 microns, greater than or equal to about 5 microns and less than orequal to about 10 microns, greater than or equal to about 5 micron andless than or equal to about 40 microns, greater than or equal to about 5microns and less than or equal to about 20 microns, or any value in theranges/sub-ranges defined by these values. Platelets having a lengthand/or width that is less than about 5-10 times the thickness of theplatelet, such as, for example having a length and/or width that isequal to the thickness of the platelet can be oriented along their edgesin the printing ink or pigment. This can be disadvantageous sincepigment or printing ink comprising platelets that are oriented alongtheir edges may not exhibit the desired colors in reflection andtransmission modes. Dimensions such as, thicknesses, lengths and/orwidths outside these ranges are also possible.

FIG. 2A-1 illustrates an example of a platelet 20. The opticalstructure, 10 is fractured, cut, diced or otherwise separated to obtainthe separate, for example, microns sized, pieces or platelets. In someimplementations, the obtained platelets may be surrounded by anencapsulating layer 21. The encapsulating layer 21 can comprise amoisture resistant material, such as, for example silicon dioxide. Theencapsulating layer 21 can also comprise silica spheres 22 and 23. Thesilica spheres 22 and 23 can be of the same size or have differentsizes. The encapsulating layer 21 can help protect the at least twometal layers 13 and 15 from corrosion. The encapsulating layer 21 canadditionally and/or alternatively reduce the occurrence of delaminationof the at least two metal layers 13 and 15 from the other layers of theoptical structure 10. The optical structures 10 surrounded by theencapsulating layer 21, and potentially comprising the silica spheres 22and 23, can be configured as platelets 20 that are suitable for apigment or printing ink. The silica spheres 22 and 23 of theencapsulating layer 21 can help prevent the platelets from adhering toone another. Without the spheres the platelets may stick together liketwo microscope slides stick together. The spheres 22 and 23 can alsoprevent the platelets 20 from sticking to the print rollers in theprinting machine. One method of surrounding the optical structure 10with an encapsulating layer 21 can rely on sol-gel technology usingtetraethylorthosilicate (TEOS). In one method of forming theencapsulating layer 21, an alcohol based solution of TEOS can be addedin small quantities (e.g., one or more drops at a time) to a dispersionof the platelets in alcohol or water. A catalyst, such as, for example,an acid or sodium hydroxide solution can be added into the a dispersionof the platelets in alcohol or water in small quantities (e.g., one ormore drops at a time). The dispersion of the platelets in alcohol orwater can be heated to a temperature of about 50-70° C., while stirringto transform TEOS to a silica coating. Other processes, however, may beemployed.

In some embodiments, a plurality of platelets 20 can form a pigment.Such a pigment may be color shifting (e.g., the color reflected and/ortransmitted changes with angle of view or angle of incidence of light),in some cases. In some embodiments, non-color shifting pigment or dyemay be mixed with the pigment. In some embodiments other materials maybe included with the platelets 20 to form the pigment. Although some ofthe pigments discussed herein can provide color shift with change inviewing angle or angle of incidence of light, pigments that do notexhibit color shift with change in viewing angle or angle of incidenceof light or that produce very little color shift with change in viewingangle or angle of incidence of light are also contemplated.

In some embodiments, the platelets 20 can be added to a medium such as apolymer 25 (e.g., a polymeric resin) to form a dichroic ink, a pigment,or paint as shown in FIG. 2B-1. The platelets can be suspended in themedium (e.g., polymer) 25. The platelets can be randomly oriented in themedium (e.g., polymer) 25 as shown in FIG. 2B-1. During the printingprocess, in some cases, the individual platelets can be orientedparallel to the surface of the object (e.g., paper) to which thepigment, the paint, or the dichroic ink is being applied as a result of,for example, the printing action, gravity, and/or surface tension of thenormal drying process of the pigment, the paint, or the dichroic ink asshown in FIG. 2B-2. The medium 25 can comprise material including butnot limited to acrylic melamine, urethanes, polyesters, vinyl resins,acrylates, methacrylate, ABS resins, epoxies, styrenes and formulationsbased on alkyd resins and mixtures thereof. In some implementations, themedium 25. e.g., polymer, can have a refractive index that closelymatches the refractive index of the encapsulating silica layer 21 and/orsilica balls such that the encapsulating layer and/or the silica ballsdo not adversely affect the optical performance of the pigment, thepaint, or the dichroic ink in the medium.

In various implementations, the platelets 20 need not be surrounded byan encapsulating layer. In such implementations, one or more platelets20 that are not encapsulated by an encapsulating layer can be added ormixed with an ink or a pigment medium (e.g., varnish, polymeric resin,etc.) to obtain a dichroic ink or pigment as discussed above. In variousimplementations, the dichroic ink or pigment can comprise a plurality ofplatelets 20. The optical structures 10 that are configured as theplurality of platelets 20 can have different distributions of shapes,sizes, thicknesses and/or aspect ratios. The optical structures 10 thatare configured as the plurality of platelets 20 can also have differentoptical properties. For example, the optical structures 10 that areconfigured as the plurality of platelets 20 can also have differentcolor properties.

In some implementations, an optical structure comprising only the metallayers 13 and 15 and the transparent dielectric layer 14 without thehigh refractive index dielectric layers 12 and 16 as depicted in FIG.2A-2 can be configured as platelets as discussed above and dispersed inthe medium 25 as shown in FIG. 2B-2 to manufacture a dichroic printingink, paint or pigment as discussed above. In some implementations, theplatelets including an optical structure comprising only the metallayers 13 and 15 and the transparent dielectric layer 14 without thehigh refractive index dielectric layers 12 and 16 need not beencapsulated in an encapsulating layer as discussed above.

A silane coupling agent can be bonded to the encapsulating layer 21 toform a functionalized platelet 30 as shown in FIG. 3. Bonding of thesilane coupling agent to the encapsulating layer can occur through ahydrolyzing reaction. The silane coupling agent can bind to the polymer(e.g., polymeric resin) of the printing ink or paint medium so that theheterogeneous mixture of pigment and the polymer do not separate duringthe printing process and substantially function in much the same way asa homogeneous medium would function. The printing ink or paint mediumcan comprise material including but not limited to acrylic melamine,urethanes, polyesters, vinyl resins, acrylates, methacrylate, ABSresins, epoxies, styrenes and formulations based on alkyd resins andmixtures thereof. The silane coupling agents used can be similar to thesilane coupling agents sold by Gelest Company (Morristown, Pa. USA). Insome implementations, the silane coupling agent can comprise ahydrolyzable group, such as, for example, an alkoxy, an acyloxy, ahalogen or an amine. Following a hydrolyzing reaction (e.g.,hydrolysis), a reactive silanol group is formed, which can condense withother silanol groups, for example, with the silica spheres of theencapsulating layer 21 or the encapsulating layer of silica to formsiloxane linkages. The other end of the silane coupling agent comprisesthe R-group 31. The R-group 31 can comprise various reactive compoundsincluding but not limited to compounds with double bonds, isocyanate oramino acid moieties. Reaction of the double bond via free radicalchemistry can form bonds with the ink polymer(s) such as those based onacrylates, methacrylates or polyesters based resins. For example,isocyanate functional silanes, alkanolamine functional silanes andaminosilanes can form urethane linkages.

Without any loss of generality, in various implementations of theoptical structure 10 configured as a platelet that do not comprise theencapsulating layer, the silane coupling agent can be bonded to one orboth of the high refractive index dielectric layers 12 and 16 comprisinga dielectric material (e.g., TiO₂) suitable to be bonded with the silanecoupling agent.

Without any loss of generality, the optical structure 10 can beconsidered as an interference stack or cavity. Ambient light incident onthe surface of the optical structure 10 is partially reflected from thevarious layers of the optical structure 10 as shown by rays 47 and 48 inFIG. 4 and partially transmitted through the various layers of theoptical structure 10 as shown by ray 49 in FIG. 4. FIG. 4 illustrates anembodiment of an optical structure 10 comprising the high refractiveindex dielectric layer 12 and 16, metal layers 13 and 15 and adielectric layer 14 encapsulated in the encapsulating layer 21. Somewavelengths of the ambient light reflected from the various layers mayinterfere constructively and some other wavelengths of the ambient lightreflected from the various layers may interfere destructively.Similarly, some wavelengths of light transmitted through the variouslayers may interfere constructively and some other wavelengths of theambient light transmitted through the various layers may interferedestructively. As a result of which, the optical structure 10 appearscolored when viewed in transmission and reflection mode. In general, thecolor and the intensity of light reflected by and transmitted throughthe optical structure 10 can depend on the thickness and the material ofthe various layers of the optical structure 10. By changing the materialand the thickness of the various layers, the color and intensity oflight reflected by and transmitted through the optical structure 10 canbe varied. Without subscribing to any particular scientific theory aboutthe operation of the optical structures 10, in general, the material andthe thickness of the various layers can be configured such that some orall of the ambient light reflected by the various layers interfere suchthat a node 45 in the field 42 occurs at the two metal layer 13 and 15.Without subscribing to any particular scientific theory, it is notedthat in some cases those wavelengths that are substantially equal to thethickness of the spacer layer (e.g., wavelengths within about ±10% ofthe thickness of the spacer layer) will interfere such that a node 45 inthe field 42 occurs at the two metal layer 13 and 15. For otherwavelengths, a node 45 might not occur. Accordingly, in someimplementations, the two metal layers 13 and 15 might not be visible inthe reflection mode. Again, without subscribing to a particularscientific theory, based on the thickness of the two metal layers 13 and15 and the transparent dielectric layer 14, a portion of the incidentlight may be transmitted through the optical structure 10 as a result ofthe phenomenon of “induced transmittance” or “induced transmission”. Thereflection and transmission spectral characteristics are discussedbelow.

FIG. 5A shows a spectral plot in both transmission (curve 501 a) andreflection (curve 503 a) for a first example of the optical structure10. The materials of the various layers of the first example of theoptical structure 10 and the thickness of the various layers of thefirst example of the optical structure 10 are provided in Table 1 below.As indicated in Table 1, the first example of the optical structure 10comprises two metal layers comprising silver. The two silver layerscorrespond to the at least two metal layer 13 and 15 of the opticalstructure 10 shown in FIG. 1. Both the silver layers have the samethickness of 25 nm. A dielectric layer having a thickness of 300 nm issandwiched between the two silver layers. The dielectric layer comprisesSiO₂ which has a refractive index of 1.47011. The dielectric layercomprising SiO₂ corresponds to the transparent layer 14 having a lowrefractive index (i.e., refractive index less than 1.65). A layer ofZrO₂ is disposed on the side of each of the two silver layers that isopposite the side facing the SiO₂ layer. Each of the two layerscomprising ZrO₂ has a thickness of 150 nm. As noted from Table 1 below,ZrO₂ has a refractive index of 2.27413. The two layers comprising ZrO₂corresponds to the transparent layers 12 and 16 having a high refractiveindex (i.e., refractive index greater than or equal to 1.65). The firstexample of the optical structure 10 is encapsulated in a SiO₂ matrix asindicated in Table 1. The SiO₂ matrix is used to simulate the printingmedium or ink which has a similar refractive index.

The transmission and reflection of light observed at an angle of 0degrees with respect to a normal to the first example of the opticalstructure 10 is shown in FIG. 5A. The reflection spectrum 503 a(indicated as curve #1 in FIG. 5A) and the transmission spectrum 501 a(indicated as curve #0 in FIG. 5A) in the spectral range between about400 nm and about 700 nm which includes the visible spectral range wereobtained using a simulation software from http://thinfilm.bansteen.net.

TABLE 1 Parameters of a first example of the optical structure that hasthe reflection and transmission spectra as shown in FIG. 5A. ParametersCurve #0 # # Slab: # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # ZRO2 d =1.5e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # AG d-2.5e−08 N = (0.173038 ,1.94942) mynkdb/AG.NK # SIO2 d = 3e−07 N = (1.47011 , 0) mynkdb/SIO2.NK# AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # ZRO2 d =1.5e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # SIO2 N = (1.47011 , 0)mynkdb/SIO2.NK # # Beam: # Wavelength = (4e−07, 0) Angle = 0.0174533Polarization = 1 N = (1.47011, 0) # # Supported spectral range: 2.5e−07m-8.5e−07 m. #---------------------------------------------------------------------------------------# Lambda[nm] R[ ] #---------------------------------------------------------------------------------------Curve #1 # # Slab: # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # ZRO2 d =1.5e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # AG d = 2.5e−08 N = (0.173038, 1.94942) mynkdb/AG.NK # SIO2 d = 3e−7 N = (1.47011 , 0) mynkdb/SIO2.NK# AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK # ZRO2 d =1.5e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # SIO2 N = (1.47011 , 0)mynkdb/SIO2.NK # # Beam: # Wavelength = (4e−07, 0) Angle = 0.0174533Polarization = 1 N = (1.47011, 0) # # Supported spectral range: 2.5e−07m-8.5e−07 m. #---------------------------------------------------------------------------------------# Lambda[nm] T[ ] #---------------------------------------------------------------------------------------

It can be seen from FIG. 5A that the transmission curve 501 a (curve#0)has a peak with a maximum value occurring at a wavelength of about 520nm and the reflection curve 503 a has two peaks with a first maximumvalue occurring at a wavelength of 420 nm and a second maximum valueoccurring at a wavelength of about 650 nm. The maximum value of thetransmission and reflection peaks is greater than 0.5 which indicatesthat the transmission and reflection peaks have high intensities.Furthermore, the transmission and reflection peaks have a bandwidth asmeasured at 50% of the maximum value of the peak greater than about 20nm. The bandwidth as measured at 50% of the maximum value of the peak isreferred to as full width at half maximum (FWHM). It is observed fromFIG. 5A that the FWHM of the transmission peak is about 75 nm.

Based on the position of the transmission and reflection peaks and thebandwidth of the transmission and reflection peaks, the opticalstructure 10 can be perceived as having a first color in the reflectionmode and a second color in the transmission mode by an average humaneye. In some cases, the first color and the second color can becomplimentary colors. In some cases, the transmission and reflectionpeaks comprising a range of wavelengths of the visible spectral rangecan have a high intensity and a FWHM greater than 2 nm (e.g., FWHMgreater than or equal to about 10 nm. FWHM greater than or equal toabout 20 nm, FWHM greater than or equal to about 30 nm. FWHM greaterthan or equal to about 40 nm, FWHM greater than or equal to about 50 nm,FWHM greater than or equal to about 60 nm, FWHM greater than or equal toabout 70 nm. FWHM greater than or equal to about 100 nm, FWHM greaterthan or equal to about 200 nm, FWHM less than or equal to about 300 nm.FWHM less than or equal to about 250 nm, or any value in arange/sub-range defined by these values).

The one or more reflection peaks can be considered to have a highintensity if the reflectivity or reflectance of the peak in a range ofvisible wavelengths is greater than or equal to about 50% and less thanor equal to about 100%. For example, the one or more reflection peakscan be considered to have a high intensity if the amount of lightreflected or reflectivity or reflectance in a range of visiblewavelengths is greater than or equal to about 55% and less than or equalto about 99%, greater than or equal to about 60% and less than or equalto about 95%, greater than or equal to about 70% and less than or equalto about 90%, greater than or equal to about 75% and less than or equalto about 85%, or any value in a range/sub-range defined by these values.

The one or more transmission peaks can be considered to have a highintensity if the transmissivity or transmittance of the peak in a rangeof visible wavelengths is greater than or equal to about 50% and lessthan or equal to about 100%. For example, the one or more transmissionpeaks can be considered to have a high intensity if the amount of lighttransmitted or transmissivity or transmittance in a range of visiblewavelengths is greater than or equal to about 55% and less than or equalto about 99%, greater than or equal to about 60% and less than or equalto about 95%, greater than or equal to about 70% and less than or equalto about 90%, greater than or equal to about 75% and less than or equalto about 85%, or any value in a range/sub-range defined by these values.

The first example of the optical structure 10 having a design asdepicted in Table 1 and having a reflection spectrum and a transmissionspectrum as shown in FIG. 5A appears green in transmission mode and asmagenta in reflection mode to an average human eye. Without any loss ofgenerality, it can be advantageous, in various implementations, for thepeaks in the reflection and transmission spectra to be non-overlappingas shown in FIGS. 5A and 5B such that a reflection peak having a highestpossible reflectance or reflectivity can be obtained in one region ofthe visible spectral range and a transmission peak having a highestpossible transmittance or transmissivity can be obtained in anon-overlapping region of the visible spectral range. Accordingly, thereflected color and the transmitted color can be different andpotentially complementary to each other, such as, for example, red andgreen, yellow and violet, blue and orange, green and magenta, etc.

The shape of the transmission and reflection peaks, the position of themaximum of the transmission and reflection peaks, the FWHM of thetransmission and reflection peaks, etc. can be varied by varying thematerials and/or thickness of the various layers of the opticalstructure 10. This can be observed from FIG. 5B which depicts thereflection spectrum 503 b and transmission spectrum 501 b of a secondexample of the optical structure 10 which has the same materialcomposition as the first example of the optical structure 10 butdifferent thickness for the various layers. The parameters of the secondexample of the optical structure 10 are provided in Table 2 below. Asnoted from Table 2, the thickness of the dielectric layer comprisingSiO₂ and having a refractive index of 1.47011 in the second example ofthe optical structure 10 is 400 nm instead of 300 nm in the firstexample of the optical structure 10. Furthermore, the thickness of thetwo ZrO₂ disposed on either side of each of the two silver layers is 225nm in the second example of the optical structure 10 instead of 150 nmin the first example of the optical structure 10.

TABLE 2 Parameters of a second example of the optical structure that hasthe reflection and transmission spectra as shown in FIG. 5B. ParametersCurve #0 # # Slab: # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # ZRO2 d =2.25e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # AG d = 2.5e−08 N = (0.173038, 1.94942) mynkdb/AG.NK # SIO2 d = 4e−07 N = (1.47011 , 0)mynkdb/SIO2.NK # AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK #ZRO2 d = 2.25e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # SIO2 N = (1.47011 ,0) mynkdb/SIO2.NK # # Beam: # Wavelength = (4e−07, 0) Angle = 0.0174533Polarization = 1 N = (1.47011, 0) # # Supported spectral range: 2.5e−07m-8.5e−07 m. #---------------------------------------------------------------------------------------# Lambda[nm] R[ ] #---------------------------------------------------------------------------------------Curve #1 # # Slab: # SIO2 N = (1.47011 , 0) mynkdb/SIO2.NK # ZRO2 d =2.25e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # AG d = 2.5e−08 N = (0.173038, 1.94942) mynkdb/AG.NK # SIO2 d = 4e−07 N = (1.47011 , 0)mynkdb/SIO2.NK # AG d = 2.5e−08 N = (0.173038 , 1.94942) mynkdb/AG.NK #ZRO2 d = 2.25e−07 N = (2.27413 , 0) mynkdb/ZRO2.NK # SIO2 N = (1.47011 ,0) mynkdb/SIO2.NK # # Beam: # Wavelength = (4e−07, 0) Angle = 0.0174533Polarization = 1 N = (1.47011, 0) # # Supported spectral range: 2.5e−07m-8.5e−07 m. #---------------------------------------------------------------------------------------# Lambda[nm] T[ ] #---------------------------------------------------------------------------------------

As a result of the change in the thickness of the dielectric layerscomprising SiO₂ and ZrO₂ between the second example of the opticalstructure and the first example of the optical structure, an average eyewould perceive the second example of the optical structure to appeargreen in reflection mode and a magenta in transmission mode when viewedalong a direction normal to the surface of the second example of theoptical structure.

The color of the first example and the second example of the opticalstructure 10 as perceived by the average human eye in reflection modeand transmission mode can shift from the above described magenta andgreen colors at different viewing angles with respect to the normal tothe surface of the first example and the second example of the opticalstructure 10. For example, the first example of the optical structure 10can appear yellowish green in reflection mode and blue in transmissionmode when viewed at an angle of about 35 degrees with respect to thenormal to the surface of the first example of the optical structure 10.As another example, the second example of the optical structure 10 canappear pale purple in reflection mode and yellowish in transmission modewhen viewed at an angle of about 35 degrees with respect to the normalto the surface of the second example of the optical structure 10.Without any loss of generality, the reflection and the transmissionpeaks can exhibit a blue shift towards shorter wavelengths as theviewing angle with respect to the normal to the surface of the firstexample and the second example of the optical structure 10 increases.

TABLE 3 CIELab values for transmission mode when the first example ofthe optical structure having parameters as described in Table 1 isviewed at different viewing angles in the presence of a D65 lightsource. Incident Angle L* a* b* 0.0 66.0433 −91.9989 11.4335 Design:First Example of 5.0 65.5578 −91.5328 9.3070 the Optical Structure 10.064.0035 −89.0283 2.6936 Polarization: P 15.0 61.1497 −81.1844 −8.9303Source: D65 20.0 56.8304 −63.3282 −25.7758 Observer: CIE 1931 25.051.2146 −32.8229 −46.6651 Mode: Transmittance 30.0 44.8902 5.7777−67.7337 35.0 38.6590 39.5335 −81.9630 40.0 33.4474 53.5162 −81.665245.0 30.4059 43.0007 −64.1869

TABLE 4 CIELab values for reflection mode when the first example of theoptical structure having parameters as described in Table 1 is viewed atdifferent viewing angles in the presence of a D65 light source. IncidentAngle L* a* b* 0.0 79.2753 51.6407 −11.0765 Design: First Example of 5.079.6541 50.6966 −9.6957 the Optical Structure 10.0 80.8290 47.4222−5.3025 Polarisation: P 15.0 82.8379 40.8204 2.7687 Source: D65 20.085.5358 30.2258 15.3945 Observer: CIE 1931 25.0 88.5026 16.2157 33.3659Mode: Reflectance 30.0 91.2316 1.0176 55.5312 35.0 93.4068 −11.016970.1468 40.0 94.9289 −14.7597 57.7563 45.0 95.7892 −10.6419 32.4479

Tables 3 and 4 above provide the CIELa*b* values for transmission modeand reflection mode respectively when the first example of the opticalstructure having parameters as described in Table 1 is viewed atdifferent viewing angles in the presence of a D65 light source. Tables 5and 6 below provide the CIELa*b* values for transmission mode andreflection mode respectively when the second example of the opticalstructure having parameters as described in Table 2 is viewed atdifferent viewing angles in the presence of a D65 light source. TheCIELab color closely represent the colors perceived by an average humaneye. The CIELab color space mathematically describe various colorsperceived by an average human eye in the three dimensions L forlightness, a for the color component green-red, and b for the colorcomponent from blue-yellow. The a-axis extends longitudinally in a planefrom green (represented by −a) to red (represented by +a). The b-axisextends along a transverse direction in the plane perpendicular to thea-axis from blue (represented by −b) to yellow (represented by +b). Thebrightness is represented by the L-axis which is perpendicular to thea-b plane. The brightness increases from black represented by L=0 towhite represented by L=100. The CIELab values for different viewingangles using a D65 illuminant were calculated using Essential MacleodThin Film Software.

TABLE 5 CIELab values for transmission mode when the second example ofthe optical structure having parameters as described in Table 2 isviewed at different viewing angles in the presence of a D65 lightsource. Incident Angle L* a* b* 0.0 35.3624 87.7761 −73.0966 Design:Second Example of 5.0 35.9375 88.1214 −71.4170 the Optical Structure10.0 37.8504 88.3232 −65.5105 Polarization: P 15.0 41.5481 86.2320−53.1339 Source: D65 20.0 47.3489 79.0290 −32.0276 Observer: CIE 193125.0 54.8227 62.6584 −2.6495 Mode: Transmittance 30.0 62.6567 31.673029.2861 35.0 68.8117 −13.6155 53.1104 40.0 70.1939 −60.8762 56.3246 45.063.8734 −83.2865 29.4710

TABLE 6 CIELab values for reflection mode when the second example of theoptical structure having parameters as described in Table 2 is viewed atdifferent viewing angles in the presence of a D65 light source. IncidentAngle L* a* b* 0.0 95.0631 −31.7647 48.4548 Design: Second Example of5.0 94.9402 −32.7902 47.4892 the Optical Structure 10.0 94.5010 −35.811843.8268 Polarisation: P 15.0 93.5195 −40.5801 35.7606 Source: D65 20.091.6012 −45.9635 22.4005 Observer: CIE 1931 25.0 88.3120 −46.8681 5.3389Mode: Reflectance 30.0 83.5384 −31.2961 −12.0407 35.0 78.2978 5.6475−26.1375 40.0 76.3297 41.2278 −30.5320 45.0 81.1875 43.5513 −17.6926

The optical performance of two additional examples of optical structureshaving parameters provided in Tables 7 and 8 were analyzed. Theadditional examples of optical structures were designed using EssentialMacleod Thin Film Software. The material composition and the thicknessof the various layers for the third example of the optical structure areprovided in Table 7 and the material composition and the thickness ofthe various layers for the fourth example of the optical structure areprovided in Table 8.

TABLE 7 Material Composition and thickness of the various layers of thethird example of the optical structure 10. Optical Physical Thickness(Full Wavelength Refractive Extinction Optical Thickness Layer MaterialIndex Coefficient Thickness) (nm) SiO2 1.46180 0.00000 1 ZrO2 1.000002.06577 0.00004 1.00000000 246.88 2 Ag 1.00000 0.05100 2.960000.00250000 25.00 3 SiO2 1.00000 1.46180 0.00000 0.50000000 174.44 4 Ag1.00000 0.05100 2.96000 0.00250000 25.00 5 ZrO2 1.00000 2.06577 0.000041.00000000 246.88 Substrate Glass 1.52083 0.00000 Total Thickness2.50500000 718.21

TABLE 8 Material Composition and thickness of the various layers of thefourth example of the optical structure 10. Optical Physical Thickness(Full Wavelength Refractive Extinction Optical Thickness Medium MaterialIndex Coefficient Thickness) (nm) SiO2 1.46180 0.00000 1 ZrO2 1.000002.06577 0.00004 0.50000000 123.44 2 Ag 1.00000 0.05100 2.960000.00250000 25.00 3 SiO2 1.00000 1.46180 0.00000 0.75000000 261.66 4 Ag1.00000 0.05100 2.96000 0.00250000 25.00 5 ZrO2 1.00000 2.06577 0.000040.50000000 123.44 Substrate Glass 1.52083 0.00000 Total Thickness1.75500000 558.55

The material composition of the various layers of the third and thefourth example of the optical structure 10 is the same as the materialcomposition of the various layers of the first and the second example ofthe optical structure 10. For example, similar to the first and thesecond example of the optical structure 10, the third and the fourthexamples of the optical structure 10 comprise a SiO₂ layer sandwiched bytwo silver layers with ZrO₂ layers disposed on the side of the twosilver layers opposite the side facing the SiO₂ layer. However, thethickness of the various layers is different for each of the first,second, third and fourth examples of the optical structure 10.

The third example of the optical structure 10 comprises two silverlayers having a thickness of 25 nm each sandwiching a dielectric layerhaving a thickness of 174.44 nm and comprising SiO₂. The third exampleof the optical structure 10 comprises a layer of ZrO₂ on the side of thesilver layers opposite the side facing the SiO₂ layer. Each ZrO₂ layerhas a thickness of 246.88 nm. The total thickness of the third exampleof the optical structure 10 is 718.21 nm.

The fourth example of the optical structure 10 comprises two silverlayers having a thickness of 25 nm each sandwiching a dielectric layerhaving a thickness of 261.66 nm and comprising SiO₂. The fourth exampleof the optical structure 10 comprises a layer of ZrO₂ on the side of thesilver layers opposite the side facing the SiO₂ layer. Each ZrO₂ layerhas a thickness of 123.44 nm. The total thickness of the fourth exampleof the optical structure 10 is 558.55 nm.

FIG. 6A illustrates the a*b* values in the CIELa*b* color space for thefirst example of the optical structure 10 having parameters as describedin Table 1 for different viewing angles between 0 degrees and 45 degreeswith respect to the normal to the surface of the first example of theoptical structure 10 in reflection mode. It is observed from FIG. 6Athat at a viewing angle of 0 degrees with respect to the normal to thesurface of the first example of the optical structure 10, the firstexample of the optical structure 10 appears magenta to an average humaneye in reflection mode. As the viewing angle increases the colorreflected by the first example of the optical structure 10 shifts alongthe curve 601 a in the direction of the arrow towards yellow.

FIG. 6B illustrates the a*b* values in the CIELa*b* color space for thesecond example of the optical structure 10 having parameters asdescribed in Table 2 for different viewing angles between 0 degrees and45 degrees with respect to the normal to the surface of the secondexample of the optical structure 10 in reflection mode. It is observedfrom FIG. 6B that at a viewing angle of 0 degrees with respect to thenormal to the surface of the second example of the optical structure 10,the second example of the optical structure 10 appears yellowish greento an average human eye in reflection mode. As the viewing angleincreases the color reflected by the second example of the opticalstructure 10 shifts along the curve 601 b in the direction of the arrowtowards magenta.

FIG. 6C illustrates the a*b* values in the CIELa*b* color space for thethird example of the optical structure 10 having parameters as describedin Table 7 for different viewing angles between 0 degrees and 45 degreeswith respect to the normal to the surface of the third example of theoptical structure 10 in reflection mode. It is observed from FIG. 6Cthat at a viewing angle of 0 degrees with respect to the normal to thesurface of the third example of the optical structure 10, the thirdexample of the optical structure 10 appears green to an average humaneye in reflection mode. As the viewing angle increases the colorreflected by the third example of the optical structure 10 shifts alongthe curve 601 c in the direction of the arrow towards blue at 350. Thetransmission color moves from red to orange as the viewing angleincreases to 350. It is noted that the various reflection andtransmission color curves move counterclockwise in the various a* b*plots of FIGS. 6A-6D and 7A-7D.

FIG. 6D illustrates the a*b* values in the CIELa*b* color space for thefourth example of the optical structure 10 having parameters asdescribed in Table 8 for different viewing angles between 0 degrees and45 degrees with respect to the normal to the surface of the fourthexample of the optical structure 10 in reflection mode. It is observedfrom FIG. 6D that at a viewing angle of 0 degrees with respect to thenormal to the surface of the fourth example of the optical structure 10,the fourth example of the optical structure 10 appears yellow to anaverage human eye in reflection mode. As the viewing angle increases thecolor reflected by the fourth example of the optical structure 10 shiftsalong the curve 601 d in the direction of the arrow towards grey. Intransmission the color seen at zero degrees is blue moving to magenta at35°. This sample is configured as a dichroic film/pigment that has avery small color shift as the angle of view changes.

FIG. 7A illustrates the a*b* values in the CIELa*b* color space for thefirst example of the optical structure 10 having parameters as describedin Table 1 for different viewing angles between 0 degrees and 45 degreeswith respect to the normal to the surface of the first example of theoptical structure 10 in transmission mode. It is observed from FIG. 7Athat at a viewing angle of 0 degrees with respect to the normal to thesurface of the first example of the optical structure 10, the firstexample of the optical structure 10 appears green to an average humaneye in transmission mode. As the viewing angle increases the colortransmitted by the first example of the optical structure 10 shiftsalong the curve 701 a in the direction of the arrow towards violet.

FIG. 7B illustrates the a*b* values in the CIELa*b* color space for thesecond example of the optical structure 10 having parameters asdescribed in Table 2 for different viewing angles between 0 degrees and45 degrees with respect to the normal to the surface of the secondexample of the optical structure 10 in transmission mode. It is observedfrom FIG. 7B that at a viewing angle of 0 degrees with respect to thenormal to the surface of the second example of the optical structure 10,the second example of the optical structure 10 appears purple to anaverage human eye in transmission mode. As the viewing angle increasesthe color reflected by the second example of the optical structure 10shifts along the curve 701 b in the direction of the arrow towardsgreen.

FIG. 7C illustrates the a*b* values in the CIELa*b* color space for thethird example of the optical structure 10 having parameters as describedin Table 7 for different viewing angles between 0 degrees and 45 degreeswith respect to the normal to the surface of the third example of theoptical structure 10 in transmission mode. It is observed from FIG. 7Cthat at a viewing angle of 0 degrees with respect to the normal to thesurface of the third example of the optical structure 10, the thirdexample of the optical structure 10 appears red to an average human eyein transmission mode. As the viewing angle increases the color reflectedby the third example of the optical structure 10 shifts along the curve701 c in the direction of the arrow towards orange.

FIG. 7D illustrates the a*b* values in the CIELa*b* color space for thefourth example of the optical structure 10 having parameters asdescribed in Table 8 for different viewing angles between 0 degrees and45 degrees with respect to the normal to the surface of the fourthexample of the optical structure 10 in transmission mode. It is observedfrom FIG. 7D that at a viewing angle of 0 degrees with respect to thenormal to the surface of the fourth example of the optical structure 10,the fourth example of the optical structure 10 appears blue to anaverage human eye in transmission mode. As the viewing angle increasesthe color reflected by the fourth example of the optical structure 10shifts along the curve 701 d in the direction of the arrow towardsmagenta.

The optical structures 10 are considered to be illuminated by D65illumination for generating the curves of FIGS. 6A-6D and 7A-7D.

FIGS. 8A and 8B respectively illustrate the transmittance andreflectance spectra for the third example of the optical structure 10having parameters as described in Table 7. As noted, from FIGS. 8A and8B, the third example of the optical structure 10 has a peaktransmittance at about 650 nm while the reflectance is substantiallyuniform in the spectral region between about 400 nm and about 600 nm anda dip around 650 nm.

FIGS. 8C and 8D respectively illustrate the transmittance andreflectance spectrum for the fourth example of the optical structure 10having parameters as described in Table 8. As noted, from FIGS. 8C and8D, the fourth example of the optical structure 10 has a peaktransmittance between about 470 nm and about 480 nm while thereflectance is substantially uniform in the spectral region betweenabout 520 nm and about 700 nm and a dip around 470 nm.

The optical performance of an additional fifth example of the opticalstructure 10 are analyzed. The fifth example of the optical structure 10comprised a glass substrate, a first dielectric layer comprising CeO₂over the substrate, a first metal layer comprising aluminum over thefirst dielectric layer, a second dielectric layer comprising CeO₂ overthe first metal layer, a second metal layer comprising aluminum over thesecond dielectric layer, and a third dielectric layer comprising CeO₂over the second metal layer. The thickness of various metal anddielectric layers can be configured to appear blue/violet intransmission at a viewing angle between about 0 degrees and about 40degrees with respect to a normal to the surface of the fifth example ofthe optical structure 10 and yellow/green in reflection at viewingangles between 0 degrees and about 40 degrees with respect to a normalto the surface of the fifth example of the optical structure 10.

FIGS. 8E and 8F respectively illustrate the transmittance andreflectance spectrum for the fifth example of the optical structure 10discussed above. FIG. 8G illustrates the a*b* values in the CIELa*b*color space for the fifth example of the optical structure 10 fordifferent viewing angles between 0 degrees and 40 degrees with respectto the normal to the surface of the fourth example of the opticalstructure 10 in transmission mode. It is observed from FIG. 8G that at aviewing angle of 0 degrees with respect to the normal to the surface ofthe fifth example of the optical structure 10, the fifth example of theoptical structure 10 appears blue to an average human eye intransmission mode. As the viewing angle increases the color reflected bythe fifth example of the optical structure 10 shifts along the curve 751a in the direction of the arrow towards violet.

FIG. 8H illustrates the a*b* values in the CIELa*b* color space for thefifth example of the optical structure 10 for different viewing anglesbetween 0 degrees and 40 degrees with respect to the normal to thesurface of the fifth example of the optical structure 10 in reflectionmode. It is observed from FIG. 8H that at a viewing angle of 0 degreeswith respect to the normal to the surface of the fifth example of theoptical structure 10, the fifth example of the optical structure 10appears yellow to an average human eye in reflection mode. As theviewing angle increases the color reflected by the fifth example of theoptical structure 10 shifts along the curve 751 b in the direction ofthe arrow towards green.

Various implementations of an optical structure that can be used as asecurity feature can comprise a dielectric region comprising one or moredielectric materials surrounded by a partially optically transmissive orpartially reflective metal layer (e.g., partially reflective andpartially transmissive metal layer). For example, the optical structurecan comprise a dielectric region having first and second major surfaces(e.g., top and bottom) and edges (or sides) therebetween. The partiallyreflective and partially transmissive metal layer can be disposed on theedges (or sides) in addition to being disposed on the first and secondmajor surfaces (e.g., top and bottom). In various implementations, thedielectric region comprising the one or more dielectric materials isoptical transmissive and in some configurations may be opticallytransparent. In certain implementations, the region comprising the oneor more dielectric materials is surrounded by a partially opticallytransmissive and partially reflective metal layer. In variousimplementations, the one or more dielectric materials can comprisepolymer, glass, oxides (e.g., SiO₂. TiO₂) or other dielectric materials.In various implementations, the dielectric region can comprise adielectric substrate coated with a one or more dielectric materials(e.g., layers) having a refractive index equal to, less than or greaterthan the refractive index of the dielectric substrate. In variousimplementations, the dielectric region can comprise a first dielectricmaterial (e.g., first dielectric layer) having a first refractive indexsurrounded by a second dielectric material (e.g., second dielectriclayer) having a second refractive index. The second refractive index canbe equal to, less than or greater than the first refractive index.

FIGS. 9A and 9B illustrate different embodiments of such opticalstructures. FIG. 9A schematically illustrates a cross-sectional view ofan embodiment of an optical structure 70 a comprising a dielectricregion 30 a surrounded by a partially reflective and partiallytransmissive metal layer 35 a. The optical structure 70 a shown in FIG.9A has a rectilinear (e.g., rectangular) cross-section. FIG. 9Bschematically illustrates a cross-sectional view of another embodimentof an optical structure 70 b comprising a dielectric region 30 bsurrounded by a partially reflective and partially transmissive metallayer 35 b. The optical structure 70 b shown in FIG. 9B has a circularcross-section.

The dielectric region 30 a and/or 30 b can comprise one or moredielectric materials such as, for example, polymer, magnesium fluoride,silicon dioxide, aluminum oxide, titanium oxide, cerium oxide, anytransparent oxide material, any transparent nitride material, anytransparent sulfide material, glass, combinations of any of thesematerials or any other inorganic or organic material. The refractiveindex of the one or more dielectric materials in the dielectric region30 a and/or 30 b can have a value between about 1.35 and about 2.5. Forexample, the refractive index of the one or more dielectric materials inthe dielectric region 30 a and/or 30 b can have a value between about1.38 and 1.48, between about 1.48 and about 1.58, between about 1.58 andabout 1.78, between about 1.75 and about 2.0, between about 2.0 andabout 2.25, between about 2.25 and about 2.5, or any value in anyrange/sub-range defined by these values. Values outside these ranges arealso possible, in some implementations. The dielectric region 30 aand/or 30 b can comprise a dielectric substrate coated with a one ormore dielectric materials having a refractive index equal to, less thanor greater than the refractive index of the dielectric substrate. Invarious implementations, the dielectric region 30 a and/or 30 b cancomprise a first dielectric material having a first refractive indexsurrounded by a second dielectric material having a second refractiveindex. The second refractive index can be equal to, less than or greaterthan the first refractive index.

In various implementations, the dielectric region 30 a and/or 30 b canbe configured as a slab, flake, a sphere, spheroid, ellipsoid, disc, orany other 3-dimensional shape enclosing a volume. The dielectric region30 a and/or 30 b may have a regular or irregular shape. For example, asshown in FIG. 9A, the dielectric region 30 a can be configured as a slabhaving two major surfaces 31 a and 31 b and one or more edge surfacesdisposed between the two major surfaces 31 a and 31 b. In someimplementations, a number of edges surfaces may be disposed between thetwo major surfaces 31 a and 31 b. The number of edge surfaces may, forexample, be one, two, three, four, five, six, seven, eight, nine, ten,twelve, twenty, thirty, fifty, etc. or in any range between any of thesevalues. Values outside these ranges are also possible. The majorsurfaces 31 a and 31 b can have a variety of shapes. For example, one orboth of the major surfaces 31 a and 31 b can have a rectilinear orcurvilinear shape in certain implementations. The shape may be regularor irregular in certain implementations. For example, one or both of themajor surfaces 31 a and 31 b can have a square shape, a rectangularshape, a circular shape, an oval shape, an elliptical shape, pentagonalshape, a hexagonal shape, an octagonal shape or any polygonal shape. Invarious implementations, one or both of the major surface 31 a and 31 bcan have jagged edges such that the lateral dimensions (e.g., length orwidth) of the one or both of the major surface 31 a and 31 b variesacross the area of the one or both of the major surface 31 a and 31 b.Other configurations are also possible. Additionally, other shapes arealso possible. One or more of the edge surfaces can have a variety ofshapes (e.g., as viewed from the side), such as, for example, a squareshape, a rectangular shape, an oval shape, an elliptical shape, apentagonal shape, a hexagonal shape, an octagonal shape or any apolygonal shape.

The shape of the one or more of the edge surfaces (e.g., as viewed fromthe side) can be rectilinear or curvilinear in certain implementations.The shape may be regular or irregular in certain implementations.Similarly, the cross-section through the dielectric region 30 a and/or30 b parallel to one of the major surfaces 31 a and 31 b, can berectilinear or curvilinear in certain implementations and can be regularor irregular in certain implementations. For example, the cross-sectioncan have a square shape, a rectangular shape, a circular shape, an ovalshape, an elliptical shape, pentagonal shape, a hexagonal shape, anoctagonal shape or any a polygonal shape. Other shapes are alsopossible. Likewise, the cross-section through the dielectric material orregion 30 a and/or 30 b perpendicular to one of the surfaces 31 a and 31b, can be rectilinear or curvilinear in certain implementations and canbe regular or irregular implementations. For example, the cross-sectioncan have a square shape, a rectangular shape, a circular shape, an ovalshape, an elliptical shape, pentagonal shape, a hexagonal shape, anoctagonal shape or any a polygonal shape. Other shapes are alsopossible. In various implementations, an area, a length and/or a widthof the major surfaces 31 a and 31 b of the dielectric region 30 a can begreater than or equal to about 2, 3, 4, 5, 6, 8, or 10 times thethickness of the dielectric region 30 a and less than or equal to about50 times the thickness of the dielectric region 30 a, or any value in arange/sub-range between any of these values. Accordingly, the dielectricregion 30 a can have a large aspect ratio.

In some implementations, a thickness (T) of the dielectric region 30 acan correspond to the distance between the two major surfaces 31 a and31 b along a vertical direction as shown in FIG. 9A. As another example,as shown in FIG. 9B, the dielectric material 30 b can be configured as asphere. A thickness of the dielectric material 30 b configured as asphere can correspond to the diameter of the sphere. In otherimplementations, the dielectric material 30 a and/or 30 b can beconfigured as a cube, a rectangular cuboid, a cylinder, an ellipsoid, anovoid or any other three-dimensional shape. The shape may be curvilinearor rectilinear in certain implementations. The shape may be regular orirregular in certain implementations. Accordingly, in someimplementations, the dielectric region 30 a and/or 30 b can beconfigured as an irregularly shaped object enclosing a volume of one ormore dielectric materials.

In various implementations, light can be transmitted through the opticalstructure 70 a or 70 b and reflected by surfaces of the opticalstructure 70 a or 70 b. Moreover, in various implementations, thedielectric region 30 a and/or 30 b can have a thickness that allowslight incident on one side of the metal layer 35 a and/or 35 b toconstructively or destructively interfere. For example, in variousimplementations, the thickness of the dielectric region 30 a and/or 30 bcan be approximately a quarter wavelength of light (e.g., visible light)incident thereon or an integer multiple of a quarter wavelength. Invarious implementations, the thickness of the dielectric region 30 aand/or 30 b may be, for example, ¼, ¾, 5/4, 7/4, 9/4, 10/4, etc. of thewavelength of visible light incident on the dielectric material 30 a or30 b. As a result various wavelengths of incident light canconstructively or destructively interfere as it is transmitted throughthe optical structure 70 a or 70 b or reflected by the optical structure70 a or 70 b. Accordingly, in some configurations, color light isreflected by and/or transmitted through the optical structure when whitelight is incident thereon. In some implementations, a first color isreflected and a second different color is transmitted when white lightis incident on the optical structure. In some case, the first color andthe second color can be complementary.

In various implementations, for example, to obtain constructiveinterference of incident visible light, a thickness (or lateraldimension) of the dielectric region 30 a and/or 30 b can have a valuebetween about 90 nm and about 2 microns. In various implementations, athickness (or lateral dimension) of the dielectric region 30 a and/or 30b can be greater than or equal to about 90 nm and less than or equal toabout 1 microns, greater than or equal to about 100 nm and less than orequal to about 1.0 microns, greater than or equal to about 300 nm andless than or equal to about 1.0 microns, greater than or equal to about400 nm and less than or equal to about 900 nm, greater than or equal toabout 500 nm and less than or equal to about 800 nm, greater than orequal to about 600 nm and less than or equal to about 700 nm, or anythickness in any range/sub-range defined by these values. Values outsidethese ranges are also possible, in some implementations.

The dielectric material 30 a and/or 30 b can be purchased from varioussuppliers (e.g., Tyndall Institute, Glassflake, Ltd., SigmaTechnologies) or custom made by synthesizing in a laboratory or amanufacturing facility. In some implementations, the optical structure70 a (or 70 b) and/or the dielectric region 30 a (or 30 b) can compriseflakes (e.g., glass flakes available from Glassflake. Ltd.http://www.glassflake.com/pages/home). In some implementations, theflakes can comprise glass such as, for example, borosilicate flakeshaving an average thickness between about 90 nm and about 2 microns(e.g., an average thickness of about 1.2 microns) that may or may not becoated with coatings (e.g., high refractive index metal oxides such asTiO₂ and/or silica). In various implementations, lateral dimensions(e.g., length and a width) of the flakes can be between about 5 micronsand about 20 microns. Values outside these ranges are also possible, insome implementations.

As discussed above, the dielectric region 30 a or 30 b can be surroundedby a partially reflective and a partially transmissive metal layer 35 aor 35 b. In some implementations, the metal layer 35 a or 35 b cancomprise a metal having a ratio of the real part (n) of the refractiveindex to the imaginary part (k) of the refractive index (k) that is lessthan 1 as discussed above. For example, the metal layer 35 a or 35 b cancomprise metals that have an n/k value between about 0.01 and about 0.6,between about 0.015 and about 0.6, between about 0.01 and about 0.5,between about 0.01 and about 0.2, between about 0.01 and about 0.1, orany value in a range or sub-range defined by these values. Valuesoutside these ranges are also possible, in some implementations.Accordingly, the metal layer 35 a or 35 b can comprise silver, silveralloys, gold, aluminum or copper and their respective alloys, nickel(Ni) and palladium (Pd).

In various implementations, a thickness of the metal layer 35 a or 35 bcan be configured such that the metal layer 35 a or 35 b is at leastpartially transmissive and partially reflective to light in the visiblespectral region between about 400 nm and about 800 nm. For example, thethickness of the metal layer 35 can be configured such that the metallayer 35 a or 35 b is at least partially transmissive to light in awavelength range between about 400 nm and about 500 nm, between about430 nm and about 520 nm, between about 450 nm and about 530 nm, betweenabout 520 nm and about 550 nm, between about 540 nm and about 580 nm,between about 550 nm and about 600 nm, between about 600 nm and about680 nm, between about 630 nm and about 750 nm, or any wavelength in arange/sub-range defined by any of these values. Values outside theseranges are also possible, in some implementations. Alternatively or inaddition, the thickness of the metal layer 35 a or 35 b can beconfigured such that the metal layer 35 a or 35 b is at least partiallyreflective to light in a wavelength range between about 400 nm and about500 nm, between about 430 nm and about 520 nm, between about 450 nm andabout 530 nm, between about 520 nm and about 550 nm, between about 540nm and about 580 nm, between about 550 nm and about 600 nm, betweenabout 600 nm and about 680 nm, between about 630 nm and about 750 nm, orany wavelength in a range/sub-range defined by any of these values.Values outside these ranges are also possible, in some implementations.

The thickness of the metal layer 35 a or 35 b can vary depending on thetype of metal. For example, in implementations of the optical structure70 a or 70 b comprising a metal (e.g., silver) layer 35 a or 35 b, thethickness of the metal (e.g., silver) layer 35 a or 35 b can be greaterthan or equal to about 10 nm and less than or equal to about 35 nm suchthat the metal (e.g., silver) layer 35 a or 35 b can be partiallytransmissive to light in the visible spectral range. In someimplementations, the thickness of the metal layer 35 a or 35 b can beless than about 10 nm or greater than about 35 nm depending possibly onthe type of metal used and the wavelength range in which transmissivityor transmittance is desired. Accordingly, in various implementations,the metal layer 35 a or 35 b can have a thickness greater than or equalto about 3 nm and less than or equal to about 40 nm. Values outsidethese ranges are also possible, in some implementations. As discussedabove, with reference to FIG. 4, the thickness of the metal layer 35 aor 35 b and the dielectric region 30 a or 30 b can be configured suchthat interference of some or all of the incident light reflected by themetal layer 35 a or 35 b and the one or more layers of the dielectricregion 30 a or 30 b can produce a node at or in the metal layer 35 a or35 b. Accordingly, the transmittance through the metal layer 35 a or 35b can be greater than the transmittance expected for a certain thicknessof the metal layer 35 a or 35 b. Without subscribing to any particularscientific theory, this effect is known as induced transmittance. As aresult of induced transmittance or induced transmission, the opticalstructure 70 a or 70 b may in some implementation, be configured toexhibit a first color in reflection mode and a second color intransmission mode.

Depending on the shape of the dielectric region 30 a or 30 b, thedielectric region 30 a or 30 b can have one or more outer surfaces. Themetal layer 35 a or 35 b can cover or substantially cover all the outersurfaces of the dielectric region 30 a or 30 b or a fraction thereof.Accordingly, in various implementations, the metal layer 35 a or 35 bcan be disposed over at least 50% of the one or more outer surfaces ofthe dielectric region 30 a or 30 b. For example, metal layer 35 a or 35b can be disposed over at least 50%, over at least 60%, at least 75%, atleast 80%, at least 85%, at least 90%, at least 95%, at least 99%, 100%,or any range between any of these values of the one or more outersurfaces of the dielectric region 30 a or 30 b. In some implementations,the metal layer 35 a or 35 b can be disposed over the entire area (e.g.,100%) of the one or more outer surfaces of the dielectric region 30 a or30 b. Without subscribing to any particular theory, the opticalproperties of the optical structure 70 a or 70 b can vary based on theamount of outer surface of the dielectric region 30 a or 30 b that iscovered by the metal layer 35 a or 35 b. For example, the reflectivityor reflectance and/or the transmissivity or transmittance of the opticalstructure 70 a or 70 b can vary based on the amount of outer surface ofthe dielectric region 30 a or 30 b that is covered by the metal layer 35a or 35 b.

In various implementations, the shape of the metal layer 35 a or 35 bcan conform to the shape of the underlying dielectric material 30 a or30 b. For example, in the optical structure 70 a shown in FIG. 9A, thedielectric material 30 a has a rectangular cross-section. Accordingly,the metal layer 35 a which is disposed over the major surfaces 31 a and31 b and the edge surfaces also has a rectangular cross-section. Asanother example, in the optical structure 70 b shown in FIG. 9B, thedielectric material 30 b has a circular cross-section. Accordingly, themetal layer 35 b which is disposed over the circumference of thedielectric material 30 b also has a circular cross-section. However, inother implementations, the shape of the metal layer 35 a or 35 b can bedifferent from the shape of the underlying dielectric material 30 a or30 b.

In various implementations, the optical structure 70 a or 70 bcomprising a dielectric region 30 a or 30 b surrounded by a metal layer35 a or 35 b can be configured as particles, slabs, filaments, flakes,beads (e.g., spherical beads) or platelets as discussed above. In someimplementations, the optical structure 70 a or 70 b comprising adielectric region 30 a or 30 b surrounded by a metal layer 35 a or 35 bcan have the same shape as the shape of the dielectric region 30 a or 30b. For example, the optical structure 70 a can be configured as a cubeor a rectangular cuboid when the dielectric region 30 a is configured asa cube or a rectangular cuboid as shown in FIG. 9A. As another example,the optical structure 70 b can be configured as a sphere when thedielectric region 30 b is configured as a sphere as shown in FIG. 9B. Insome cases, the optical structure 70 a or 70 b configured as a particle,a slab, a flake, a filament, or a platelet can be suitable for a pigmentor a printing ink. In some implementations, the optical structure 70 aor 70 b configured as a particle, a slab, a flake, a filament, or aplatelet can have an area (or a lateral dimension) that is about 5 to 10times or more the thickness of the optical structure 70 a or 70 bconfigured as a particle, a slab, a flake, a filament, or a platelet.Accordingly, an optical structure 70 a or 70 b configured as a particle,a slab, a flake, a filament, or a platelet can have a thickness betweenabout 100 nm and about 1 micron. In some such implementations, the area(or a lateral dimension) can be greater than or equal to about 500 nmand less than or equal to about 1 micron, greater than or equal to about1 micron and less than or equal to about 5 microns, greater than orequal to about 5 microns and less than or equal to about 10 microns,greater than or equal to about 5 micron and less than or equal to about40 microns, greater than or equal to about 5 microns and less than orequal to about 20 microns, or any value in the ranges/sub-ranges definedby these values. In various embodiments, the optical structure 70 a or70 b configured as a particle, a slab, a flake, a filament, or aplatelet can be configured such that an area, a length and/or a width ofa major surface of the optical structure 70 a or 70 b is greater than orequal to about 2, 3, 4, 5, 6, 8, or 10 times the thickness of theoptical structure 70 a or 70 b and less than or equal to about 50 timesthe thickness of the optical structure 70 a or 70 b or any value in anyrange formed by any of these values.

In various implementations, surrounding the dielectric region 30 a or 30b with the metal layer 35 a or 35 b can advantageously increase thereflectivity or reflectance of the dielectric material 30 a or 30 b atone or more wavelengths of the visible spectral range in someimplementations. In some implementations, surrounding the dielectricmaterial 30 a or 30 b with the metal layer 35 a or 35 b canadvantageously enhance or change the color appearance of the dielectricmaterial 30 a or 30 b at one or more wavelengths of the visible spectralrange in reflection and transmission mode.

In various implementations, the optical structure 70 a or 70 bcomprising the dielectric region 30 a or 30 b surrounded by the metallayer 35 a or 35 b can have a reflection spectrum with one or morereflection peaks in the visible spectral region and a transmissionspectrum with one or more transmission peaks in the visible spectralregion. Without any loss of generality, the one or more reflection peaksand the one or more transmission peak do not overlap with each other.Accordingly, the optical structure 70 a or 70 b comprising thedielectric region 30 a or 30 b surrounded by the metal layer 35 a or 35b can have a first color in the reflection mode and a second colordifferent from the first color in the transmission mode. In certainimplementations, the first color and the second color can becomplementary colors, such as, for example, red and green, yellow andviolet, blue and orange, green and magenta, etc.

In various implementations, there may be little to no shift in the firstcolor in the reflection mode for any viewing angle between a first anglewith respect to a normal to the surface of the optical structure 70 a or70 b and a second angle with respect to a normal to the surface of theoptical structure 70 a or 70 b. Likewise, in some implementations, theremay be little to no shift in the second color in the transmission modefor any viewing angle between a first angle with respect to a normal tothe surface of the optical structure 70 a or 70 b and a second anglewith respect to a normal to the surface of the optical structure 70 a or70 b. In various implementations, the first angle can have a valuebetween 0 degrees and 10 degrees (e.g., 0 degrees, 1 degree, 2 degrees,3 degrees, 4 degrees, 5 degrees, 6 degrees, 7 degrees, 8 degrees, 9degrees or 10 degrees). In various implementations, the second angle canhave a value between 20 degrees and 90 degrees (e.g., 20 degrees, 30degrees, 40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or90 degrees). Accordingly, for any viewing angle between a first angle(e.g., 0 degrees, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees,6 degrees, 7 degrees, 8 degrees, 9 degrees or 10 degrees) with respectto a normal to the surface of the optical structure 70 a or 70 b and asecond angle (e.g., 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60degrees, 70 degrees, 80 degrees or 90 degrees) with respect to a normalto the surface of the optical structure 70 a or 70 b, the color of theoptical structure 70 a or 70 b in the reflection mode and/or thetransmission mode may remain substantially the same. Likewise, in someimplementations, there may be little to no shift color shift in thecolor of the optical structure 70 a or 70 b in the reflection modeand/or the transmission mode for tilt of 10 degrees, 20 degrees, 30degrees, 40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or90 degrees or any value in a range/sub-range defined by any of thesevalues.

In some implementations, it may be desirable to have a color shift inthe first color in the reflection mode as the viewing angle changes froma first angle with respect to a normal to the surface of the opticalstructure 70 a or 70 b to a second angle with respect to a normal to thesurface of the optical structure 70 a or 70 b. Similarly, in variousimplementations, it may be desirable to have a color shift in the secondcolor in the transmission mode as the viewing angle changes from a firstangle with respect to a normal to the surface of the optical structure70 a or 70 b to a second angle with respect to a normal to the surfaceof the optical structure 70 a or 70 b. In various implementations, thefirst angle can have a value between 0 degrees and 10 degrees (e.g., 0degrees, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees, 6degrees, 7 degrees, 8 degrees, 9 degrees or 10 degrees). In variousimplementations, the second angle can have a value between 20 degreesand 90 degrees (e.g., 20 degrees, 30 degrees, 40 degrees, 50 degrees, 60degrees, 70 degrees, 80 degrees or 90 degrees) depending on the design.Accordingly, as the viewing angle changes from a first angle (e.g., 0degrees, 1 degree, 2 degrees, 3 degrees, 4 degrees, 5 degrees, 6degrees, 7 degrees, 8 degrees, 9 degrees or 10 degrees) with respect toa normal to the surface of the optical structure 70 a or 70 b to asecond angle with respect to a normal to the surface of the opticalstructure 70 a or 70 b and a second angle (e.g., 20 degrees, 30 degrees,40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or 90degrees) with respect to a normal to the surface of the opticalstructure 70 a or 70 b, the color of the optical structure 70 a or 70 bin the reflection mode and/or the transmission mode may change (e.g.,dark blue to light blue, purple to pink, dark green to light green,etc.). Likewise, in some implementations, there may be a shift in thecolor of the optical structure 70 a or 70 b in the reflection modeand/or the transmission mode for tilt of 10 degrees, 20 degrees, 30degrees, 40 degrees, 50 degrees, 60 degrees, 70 degrees, 80 degrees or90 degrees or any value in a range/sub-range defined by any of thesevalues.

Without subscribing to any particular theory, the one or more reflectionpeaks of the reflection spectrum of the optical structure 70 a or 70 bcomprising the dielectric region 30 a or 30 b surrounded by the metallayer 35 a or 35 b can have high reflectivity or reflectance. Forexample, the reflectivity or reflectance of the one or more reflectionpeaks can be greater than or equal to 30%, greater than or equal to 40%,greater than or equal to 50%, greater than or equal to 55%, greater thanor equal to 60%, greater than or equal to 65%, greater than or equal to70%, greater than or equal to 75%, greater than or equal to 80%, greaterthan or equal to 85%, greater than or equal to 90%, greater than orequal to 95% and less than or equal to 100%, or a value in anyrange/sub-range defined by these values.

Without subscribing to any particular theory, the one or moretransmission peaks of the transmission spectrum of the optical structure70 a or 70 b comprising the dielectric region 30 a or 30 b surrounded bythe metal layer 35 a or 35 b can have high transmissivity ortransmittance. For example, the transmissivity or transmittance of theone or more transmission peaks can be greater than or equal to 30%,greater than or equal to 40%, greater than or equal to 50%, greater thanor equal to 55%, greater than or equal to 60%, greater than or equal to65%, greater than or equal to 70%, greater than or equal to 75%, greaterthan or equal to 80%, greater than or equal to 85%, greater than orequal to 90%, greater than or equal to 95% and less than or equal to100%, or a value in any range/sub-range defined by these values.

The optical structures 70 a and 70 b comprising the dielectric region 30a or 30 b surrounded by the metal layer 35 a or 35 b can produce many orall the optical effects that are described above with reference tooptical structure 10 where the two metal layers 13 and 15 do notsurround the dielectric layer 14 (e.g., as shown in FIG. 1).

The metal layer 35 a or 35 b can be disposed around the dielectricmaterial 30 a or 30 b using a variety of chemical methods. For example,metal layer 35 a or 35 b can be disposed around the dielectric region 30a or 30 b using electroless method. Various implementations of anelectroless method of depositing the metal layer 35 a or 35 b cancomprise depositing the metal layer 35 a or 35 b without applyingelectrical current or voltage. Various metals such as, for example,gold, silver, or nickel can be deposited using electroless methods. Anexample of depositing metal layer 35 a or 35 b comprising silver aroundthe dielectric region 30 a or 30 b using an electroless method isdiscussed below. The electroless method of depositing silver can also bereferred to as electroless silver plating. Electroless silver platingcomprises immersing the dielectric region 30 a or 30 b in a silveringbath comprising chemical compounds of silver (e.g., silver nitrate,silver-ammonia compounds, sodium argento cyanide, etc.) and at least oneof ammonia, water, potassium hydroxide or sodium hydroxide. The chemicalcompounds of silver are reduced to metallic silver using a reducingagent which is added to the silvering bath. The metallic silver adheresto the exposed surfaces of the dielectric region 30 a or 30 b. Thereducing agent can comprise glucose, sucrose, invert sugar, stannouschloride, hydrazine, Rochelle salt, formaldehyde, or organic borane(e.g., dimethylamine borane in various implementations). In certainimplementations, the silvering bath and the reducing agent can besprayed on the dielectric region 30 a or 30 b. In some implementations,the outer surface of the dielectric region 30 a or 30 b can be activatedusing stannous chloride (SnCl₂) in preparation for the electrolessdeposition of the metal layer. Other methods of depositing the metallayer 35 a or 35 b on the outer surface of the dielectric region 30 a or30 b can also be used. For example, the metal layer 35 a or 35 b can bedisposed around the dielectric region 30 a or 30 b using methods suchas, for example, chemical vapor deposition (CVD), sputtering orelectroplating. In some implementations, the metal layer 35 a or 35 bcan be patterned around the dielectric region 30 a or 30 b.

In various implementation, a second dielectric region 40 a or 40 bcomprising one or more dielectric materials may be disposed around themetal coated dielectric region 30 a or 30 b. The second dielectricregion 40 a or 40 b may comprise high refractive index materials such asZrO₂. TiO₂, ZnS. ITO (indium tin oxide), CeO₂ or Ta₂O₃. In variousimplementations, the second dielectric region 40 a or 40 b may comprisedielectric materials having refractive index greater than 1.65 and lessthan or equal to 2.5. For example, the refractive index of the one ormore dielectric material in the second dielectric region 40 a or 40 bcan be greater than or equal to 1.65 and less than or equal to 1.75,greater than or equal to 1.75 and less than or equal to 1.85, greaterthan or equal to 1.85 and less than or equal to 1.95, greater than orequal to 1.95 and less than or equal to 2.05, greater than or equal to2.0 and less than or equal to 2.2, greater than or equal to 2.1 and lessthan or equal to 2.3, greater than or equal to 2.25 and less than orequal to 2.5, or any value in any range/sub-range defined by thesevalues. Other values outside these ranges are also possible in someimplementations. In various implementations, the refractive index of theone or more materials of the second dielectric region 40 a or 40 b canbe greater than the refractive index of the one or more materials of thedielectric region 30 a or 30 b. The thickness of the second dielectricregion 40 a or 40 b can be between 75 nm and 700 nm. For example, thethickness of the second dielectric region 40 a or 40 b can be greaterthan or equal to 75 nm and less than or equal to 100 nm, greater than orequal to 100 nm and less than or equal to 150 nm, greater than or equalto 150 nm and less than or equal to 200 nm, greater than or equal to 200nm and less than or equal to 250 nm, greater than or equal to 300 nm andless than or equal to 350 nm, greater than or equal to 400 nm and lessthan or equal to 450 nm, greater than or equal to 450 nm and less thanor equal to 500 nm, greater than or equal to about 500 nm and less thanor equal to 650 nm, greater than or equal to 650 nm and less than orequal to 700 nm, or any value in any range/sub-range defined by thesevalues. The second dielectric region 40 a or 40 b can be disposed tocover at least 50% of the outer surface of the metal layer 35 a or 35 b.For example, the second dielectric region 40 a or 40 b can be disposedto cover at least 80%, at least 90%, at least 95%, or 100% of the outersurface of the metal layer 35 a or 35 b, or any value in arange/sub-range defined by these values.

The reflected color and/or the transmitted color of the opticalstructure 70 a or 70 b comprising the second dielectric region 40 a or40 b surrounding the metal coated dielectric region 30 a or 30 b can bedifferent from the reflected color and/or the transmitted color of theoptical structure 70 a or 70 b comprising only the metal coateddielectric region 30 a or 30 b. For example, the reflected color and/orthe transmitted color of the optical structure 70 a or 70 b comprisingthe second dielectric region 40 a or 40 b surrounding the metal coateddielectric region 30 a or 30 b can be more vibrant than the reflectedcolor and/or the transmitted color of the optical structure 70 a or 70 bcomprising the metal coated dielectric region 30 a or 30 b without thesecond dielectric region 40 a or 40 b having suitable thickness and/ormaterials with suitable refractive index. The shape of the transmissionand/or reflection peaks, the position of the maximum of the transmissionand/or reflection peaks and/or the width (e.g., full width at halfmaximum (FWHM)) of the transmission and/or reflection peaks of theoptical structure 70 a or 70 b comprising the second dielectric region40 a or 40 b surrounding the metal coated dielectric region 30 a or 30 bcan be different from the shape of the transmission and/or reflectionpeaks, the position of the maximum of the transmission and/or reflectionpeaks and/or the width of the transmission and reflection peaks of theoptical structure 70 a or 70 b comprising the metal coated dielectricregion 30 a or 30 b without the second dielectric region 40 a or 40 bhaving suitable thickness and/or materials with suitable refractiveindex. For example, the width of one or more of the reflection peaks ofthe optical structure 70 a or 70 b comprising the second dielectricregion 40 a or 40 b surrounding the metal coated dielectric region 30 aor 30 b can be broader than the width of a corresponding reflection peakof the optical structure 70 a or 70 b comprising the metal coateddielectric region 30 a or 30 b without the second dielectric region 40 aor 40 b having suitable thickness and/or materials with suitablerefractive index. As another example, the width (e.g., FWHM) of one ormore of the reflection peaks of the optical structure 70 a or 70 bcomprising the second dielectric region 40 a or 40 b surrounding themetal coated dielectric region 30 a or 30 b can be greater than or equalto about 50 nm and less than or equal to about 300 nm, in someimplementations.

Various implementations of the of the optical structure 70 a or 70 bcomprising the second dielectric region 40 a or 40 b surrounding themetal coated dielectric region 30 a or 30 b can have a reflectionspectrum with one or more reflection peaks having a width (e.g., FWHM)greater than or equal to about 10 nm, greater than or equal to about 20nm, greater than or equal to about 30 nm, greater than or equal to about40 nm, greater than or equal to about 50 nm, greater than or equal toabout 60 nm, greater than or equal to about 70 nm, greater than or equalto about 100 nm, greater than or equal to about 200 nm, less than orequal to about 300 nm, less than or equal to about 250 nm, or any valuein a range/sub-range defined by these values. Various implementations ofthe optical structure 70 a or 70 b comprising the second dielectricregion 40 a or 40 b surrounding the metal coated dielectric region 30 aor 30 b can have higher reflectivity or reflectance at one or morewavelengths in the visible spectral range as compared to thereflectivity or reflectance of the optical structure 70 a or 70 bcomprising the metal coated dielectric region 30 a or 30 b without thesecond dielectric region 40 a or 40 b having suitable thickness and/ormaterials with suitable refractive index at those one or morewavelengths in the visible spectral range.

Various implementations of the of the optical structure 70 a or 70 bcomprising the second dielectric region 40 a or 40 b surrounding themetal coated dielectric region 30 a or 30 b can have a transmissionspectrum with one or more transmission peaks having a width (e.g., FWHM)greater than or equal to about 10 nm, greater than or equal to about 20nm, greater than or equal to about 30 nm, greater than or equal to about40 nm, greater than or equal to about 50 nm, greater than or equal toabout 60 nm, greater than or equal to about 70 nm, greater than or equalto about 100 nm, greater than or equal to about 200 nm, less than orequal to about 300 nm, less than or equal to about 250 nm, or any valuein a range/sub-range defined by these values.

Without subscribing to any particular theory, the one or more reflectionpeaks of the reflection spectrum of the optical structure 70 a or 70 bcomprising the second dielectric region 40 a or 40 b surrounding themetal coated dielectric region 30 a or 30 b can have high reflectivityor reflectance. For example, the reflectivity or reflectance of the oneor more reflection peaks can be greater than or equal to 30%, greaterthan or equal to 40%, greater than or equal to 50%, greater than orequal to 55%, greater than or equal to 60%, greater than or equal to65%, greater than or equal to 70%, greater than or equal to 75%, greaterthan or equal to 80%, greater than or equal to 85%, greater than orequal to 90%, greater than or equal to 95% and less than or equal to100%, or a value in any range/sub-range defined by these values.

Without subscribing to any particular theory, the one or moretransmission peaks of the transmission spectrum of the optical structure70 a or 70 b comprising the second dielectric region 40 a or 40 bsurrounding the metal coated dielectric region 30 a or 30 b can havehigh transmissivity or transmittance. For example, the transmissivity ortransmittance of the one or more transmission peaks can be greater thanor equal to 30%, greater than or equal to 40%, greater than or equal to50%, greater than or equal to 55%, greater than or equal to 60%, greaterthan or equal to 65%, greater than or equal to 70%, greater than orequal to 75%, greater than or equal to 80%, greater than or equal to85%, greater than or equal to 90%, greater than or equal to 95% and lessthan or equal to 100%, or a value in any range/sub-range defined bythese values.

Additionally, the second dielectric region 40 a or 40 b canadvantageously insulate the metal layer 35 a or 35 b from the inkvarnish when the optical structures 70 a or 70 b are configured aspigments.

In some implementations, the second dielectric region 40 a or 40 b canbe disposed around the metal coated dielectric materials 30 a or 30 busing a sol-gel process. For example, the metal coated dielectricmaterials 30 a or 30 b can be coated with a dielectric materialcomprising titanium di-oxide (TiO₂) using a sol-gel process, involvingthe hydrolysis of titanium(IV) isopropoxide. As another example, aprecursor comprising the dielectric material 40 a or 40 b is transformedto form a colloidal suspension (or a “sol”) by a series of hydrolysisand polymerization reactions. In some implementations, the colloidalsuspension comprising the dielectric material of the second dielectricregion 40 a or 40 b can be disposed on the metal coated first dielectricregion 30 a or 30 b by a coating, gelling or precipitation. The metalcoated first dielectric region 30 a or 30 b comprising the colloidalsuspension comprising the dielectric material of the second dielectricregion 40 a or 40 b can be heated or dried to obtain the metal coatedfirst dielectric region 30 a or 30 b coated with second dielectricregion 40 a or 40 b. In some implementations, the one or more materialsof the second dielectric region 40 a or 40 b can be disposed around themetal coated first dielectric region 30 a or 30 b using depositionmethods such as, for example, chemical vapor deposition method, e-beam,sputtering. In some implementations, the various deposition methods canbe combined with vibrating the metal coated first dielectric region 30 aor 30 b.

As discussed above, various embodiments of the optical structures 10, 70a or 70 b are configured to partially reflect light and partiallytransmit light. In various implementations, the reflectivity orreflectance of the optical structures 10, 70 a or 70 b at one or morewavelengths in the visible spectral range can be greater than or equalto 10%, greater than or equal to 20%, greater than or equal to 30%,greater than or equal to 40%, greater than or equal to 50%, greater thanor equal to 60%, greater than or equal to 70%, greater than or equal to80%, greater than or equal to 90%, greater than or equal to 95% and/orless than or equal to 100%, or any value in any range/sub-range definedby these value. In various implementations, the transmissivity ortransmittance of the optical structures 10, 70 a or 70 b at one or morewavelengths in the visible spectral range can be greater than or equalto 10%, greater than or equal to 20%, greater than or equal to 30%,greater than or equal to 40%, greater than or equal to 50%, greater thanor equal to 60%, greater than or equal to 70%, greater than or equal to80%, greater than or equal to 90%, greater than or equal to 95% and/orless than or equal to 100%, or any value in any range/sub-range definedby these value. In various implementations, the reflectivity orreflectance of the optical structures 10, 70 a or 70 b at one or morefirst set of wavelengths can be approximately equal to thetransmissivity or transmittance of the optical structures 10, 70 a or 70b at one or more second set of wavelengths different from the first setof wavelengths.

The optical structures 10, 70 a or 70 b can have a size, such as, forexample, a lateral dimension, an area, a length or a width of theoptical structure (e.g., a length, a width or an area of a major surfaceof the optical structure) greater than or equal to about 1 micron andless than or equal to about 50 microns. For example, the size of theoptical structures 10, 70 a or 70 b can be greater than or equal toabout 1 micron and less than or equal to 10 microns, greater than orequal to 2 microns and less than or equal to 12 microns, greater than orequal to 3 microns and less than or equal to 15 microns, greater than orequal to 4 microns and less than or equal to 18 microns, greater than orequal to 5 microns and less than or equal to 20 microns, greater than orequal to 10 microns and less than or equal to 20 microns, greater thanor equal to 15 microns and less than or equal to 25 microns, greaterthan or equal to 20 microns and less than or equal to about 30 microns,greater than or equal to 25 microns and less than or equal to 35microns, greater than or equal to 30 microns and less than or equal to40 microns, greater than or equal to 35 microns and less than or equalto 45 microns, greater than or equal to 40 microns and less than orequal to 50 microns, or a value in any range/sub-range defined by thesevalues.

The optical structures 10, 70 a or 70 b can have a size, such as, forexample, a lateral dimension, an area, a length or a width of theoptical structure (e.g., a length, a width or an area of a major surfaceof the optical structure) greater than or equal to about 1 micron andless than or equal to about 50 microns can be between 0.1 microns and2.0 microns. For example, the thickness of the optical structures 10, 70a or 70 b having a size, such as, for example, a lateral dimension, anarea, a length or a width of the optical structure (e.g., a length, awidth or an area of a major surface of the optical structure) greaterthan or equal to 0.1 micron and less than or equal to 0.3 microns,greater than or equal to 0.2 microns and less than or equal to 0.5microns, greater than or equal to 0.3 microns and less than or equal to0.6 microns, greater than or equal to 0.4 microns and less than or equalto 0.7 microns, greater than or equal to 0.5 microns and less than orequal to 0.8 microns, greater than or equal to 0.6 microns and less thanor equal to 0.9 microns, greater than or equal to 0.7 microns and lessthan or equal to 1.0 micron, greater than or equal to 1.0 micron andless than or equal to 1.2 microns, greater than or equal to 1.2 micronsand less than or equal to 1.5 microns, greater than or equal to 1.5microns and less than or equal to 2.0 microns, or a value in anyrange/sub-range defined by these values.

One or more of the optical structures 10, 70 a or 70 b discussed abovecan be incorporated with or in a document (e.g., a banknote), package,product, or other item. Optical products such as a film, a thread, alaminate, a foil, a pigment, or an ink comprising one or more of theoptical structures 10, 70 a or 70 b discussed above can be incorporatedwith or in documents such as banknotes or other documents to verifyauthenticity of the documents, packaging materials, etc. For example,the optical structures 70 a or 70 b can be configured as an ink or apigment which is disposed on a base comprising at least one of apolymer, a plastic, a paper or a fabric. The base may be flexible insome implementations. The base comprising the ink or a pigment orpigment comprising the optical structures 70 a or 70 b can be cut ordiced to obtain a thread or a foil. A plurality of optical structures10, 70 a or 70 b discussed above can be incorporated in a particularoptical product (e.g., ink, pigment, thread, filament, paper, securityink, security pigment, security thread, security filament, securitypaper, etc.). The shapes, sizes and/or aspect ratios of the plurality ofoptical structures 10, 70 a or 70 b discussed above that areincorporated in a particular optical product (e.g., ink, pigment,thread, filament, paper, security ink, security pigment, securitythread, security filament, security paper, etc.) can vary. Accordingly,a particular optical product (e.g., ink, pigment, thread, filament,paper, security ink, security pigment, security thread, securityfilament, security paper, etc.) can comprise optical structures 10, 70 aor 70 b with different distributions of shapes, sizes and/or aspectratios of the optical structures. For example, a particular opticalproduct (e.g., ink, pigment, thread, filament, paper, security ink,security pigment, security thread, security filament, security paper,etc.) can comprise optical structures 10, 70 a or 70 b with sizesdistributed around one or more mean sizes. As another example, aparticular optical product (e.g., ink, pigment, thread, filament, paper,security ink, security pigment, security thread, security filament,security paper, etc.) can comprise optical structures 10, 70 a or 70 bwith aspect ratios distributed around one or more aspect ratios.

FIG. 10 shows, for example, a banknote 80 comprising a laminated film83. The laminated film 83 comprises the optical structure 10, 70 a or 70b. The laminated film 83 can be fabricated by disposing the opticalstructure 10, 70 a or 70 b over a base or support layer or substratesuch as polymer base layer (e.g., a polyester film). The opticalstructure 10, 70 a or 70 b can be disposed over the polymer base layerby a variety of methods including but not limited to coating methods,vacuum deposition on a surface of the polymer base layer, etc. Theoptical structure 10, 70 a or 70 b may be disposed over a first side ofthe surface of the polymer base layer (e.g., polyester film). Thelaminated film 83 can be adhered to the “paper” (e.g., cellulose,cotton/linen, polymer or fabric) 81 of the banknote 80, for example, bya transparent and/or an optically clear adhesive. In various cases, asecond surface of the polymer base layer opposite the first surface ofthe base layer is disposed closer to the banknote paper 81 comprisingthe banknote and may be in contact with the adhesive. In some cases, theadhesive can be a two component adhesive with one component disposedonto the banknote paper and the other component disposed on the secondsurface of the polymer base layer opposite the first surface of the baselayer on which the optical structure 10, 70 a or 70 b is disposed. Thebanknote 80 and the laminated film 83 can be brought together forbonding. The laminated film 83 can also be attached to the banknote 80using a cross-linking thermoset adhesive. A transparent protectivebarrier coating 82 (e.g., UV curable cross-linked resin) can be disposedover the laminated film 83. The protective barrier coating 82 can extendover the edges of the laminated film 83 onto the paper (e.g., fabric) 81of the banknote. The protective barrier coating 82 can be configured toprotect the laminated film 83 against corrosion, abrasive wear andliquids that may commonly come in contact with the banknote 80 withoutsacrificing the optical effects provided by the laminated film 83. Theoptical structure 10 can be disposed facing the protective barriercoating 82 or the adhesive layer between the laminated film 83 and the“paper” 81.

In some embodiments, the optical structure 10, 70 a or 70 b can beconfigured as a thread (e.g., a windowed thread) instead of a laminatedfilm. A windowed thread can be manufactured by a variety of methods. Forexample, the thread can be woven up and down within the paper and to thesurface of the paper during the papermaking process. As another example,the windowed thread can be disposed within the paper itself so that nopart of the thread reaches the surface of the banknote. As yet anotherexample, open spaces within the paper can be provided in the regions ofthe paper comprising the thread.

The thread can be fabricated by cutting a strip of the optical structure10, for example the web, sheet, or base layer on which the layerscomprising the optical structure 10 are formed and passing the stripthrough a bath of UV curable resin. The rate at which the strip ispassed through the UV curable resin bath can be controlled to coat thesides and the edges of the strip uniformly. The strip coated with the UVcurable resin can be cured to obtain the thread. The obtained threadcomprising the optical structure 10 can be inserted (e.g., weaved) inthe banknote. In some implementations, any fringe (e.g., the jagged orragged edge of the thread) of the thread (due to hot stamping or chatterfrom any cutting operation) can be hidden from an observer by printingan opaque border around the hot stamp patch. Another way to affix theoptical structure 10, 70 a or 70 b to the banknote can include diecutting a portion of the optical structure, for example, the web, sheet,or base layer on which the layers comprising the optical structure 10,70 a or 70 b are formed and applying the portion to the banknote usingan adhesive. Various implementations of the examples of opticalstructure described above can be configured as a thread, a hot stamp, ora laminate and incorporated with or in a document (e.g., a banknote),package, product, or other item.

Without any loss of generality, the optical structure 10, 70 a or 70 bor a material (e.g., an ink, a paint or a pigment, a varnish) comprisingthe optical structure 10, 70 a or 70 b can be disposed on a basecomprising at least one of a polymer, a plastic, a paper or a fabric.The base comprising the optical structure 10, 70 a or 70 b or thematerial comprising the optical structure 10, 70 a or 70 b can be cut ordiced into a smaller portions having a variety of shapes and/or sizes.The smaller portions can be disposed on or inserted into or onto asubstrate (e.g., a bank note, paper, packaging material, fabric, etc.)using various methods. For example, the smaller portions can beconfigured as strips or threads which can be woven into the substrate.As another example, the smaller portions can be configured as foilswhich can be hot stamped on the substrate. As yet another example, thesmaller portions can be laminated to the substrate using adhesives.

FIG. 11A depicts a banknote 90 a having two transparent windows 91 a and92 a inserted into or attached on the paper (e.g., fabric) of thebanknote. Each window comprises the optical structure 10, 70 a or 70 b.In some implementations, the reflection and/or transmission spectra ofthe optical structure 10 of the window 91 a may be configured to bedifferent from the reflection and/or transmission spectra of the opticalstructure 10, 70 a or 70 b of the window 92 a. Thus, a person viewingthe banknote 90 a will perceive a first reflected color when viewing thewindow 91 a along a viewing direction (e.g., normal to the surface ofthe banknote 90 a) and a second reflected color different from the firstreflected color when viewing the window 92 a along the viewingdirection. The person may also perceive a third transmitted colordifferent from the first reflected color when viewing through the window91 a along the viewing direction. The person may additionally perceive afourth transmitted color different from the first, second and thirdcolors when viewing through the window 92 a along the viewing direction.Furthermore, upon folding the banknote 90 a over itself so that the twowindows 91 a and 92 a are at least partially aligned with respect to oneanother, the person will perceive a different color, different from thefirst, second, third and/or fourth colors in reflection and transmissionmodes when viewing the banknote 90 a along the viewing direction. Forexample, upon folding the banknote 90 a over itself so that the twowindows 91 a and 92 a are at least partially aligned with respect to oneanother, the person will perceive a reflected color that is acombination of the effects of the reflectivity or reflectance spectrumsof the two windows 91 a and 92 a and a transmitted color that is acombination of the effects of the transmission spectrums of the twowindows 91 a and 92 a. Additionally, the person can perceive color shiftof the various colors seen in the reflection and transmission modes asthe viewing angle changes. The amount of color shift may be differentfrom the different windows as well as for the combination of the twowindows.

FIG. 11B depicts an implementation of a security device 90 b (e.g., abanknote) comprising two windows 91 b and 92 b (a first and a second)inserted into or attached to the surface of the security device 90 b.The two windows 91 b and 92 b at least partially overlap in theoverlapping region 93 b. The two windows 91 b and 92 b are transparentand comprise the optical structure 10, 70 a or 70 b. The configuration(e.g., thickness or other design parameters) of the optical structures10, 70 a or 70 b in the respective windows 91 a and 91 b can be suchthat the reflection and/or transmission spectra of the optical structure10, 70 a or 70 b of the window 91 b is different from the reflectionand/or transmission spectra of the optical structure 10 of the window 92b.

Thus, a person viewing the security device 90 b along a viewingdirection (e.g., normal to the surface of the security device 90 b) willperceive (i) a first reflected color when viewing the portion of thewindow 91 b that does not overlap with the window 92 b. (ii) a secondreflected color different from the first color when viewing the portionof the window 92 b that does not overlap with the window 91 b; and (iii)a third second reflected color that is a combination of the effects ofthe reflectivity or reflectance spectrums of the two windows 91 b and 92b when viewing the overlapping region 93 b.

A person viewing the security device 90 b along a viewing direction(e.g., normal to the surface of the security device 90 b) will perceive(i) a fourth transmitted color different from the first color whenviewing through the portion of the window 91 b that does not overlapwith the window 92 b, (ii) a fifth transmitted color different from thesecond and the fourth color when viewing through the portion of thewindow 92 b that does not overlap with the window 91 b; and (iii) asixth transmitted color that is a combination of the effects of thetransmission spectrums of the two windows 91 b and 92 b when viewingthrough the overlapping region 93 b.

Additionally, in various embodiments, a person viewing the securitydevice 90 b can perceive color shift of the various colors seen in thereflection and transmission modes as the viewing angle changes. Theamount of color shift may be different from the different windows.

Although, the two windows 91 b and 92 b are shown as partiallyoverlapping in FIG. 11B, the two windows 91 b and 92 b can be completelyoverlapping. Various implementations of the security device 90 b cancomprise two or more different pigments. The two or more differentpigments can comprise optical structures 10. A respective opticalstructure of one of the two or more different pigments can havereflectance and transmittance characteristics that are different fromthe respective optical structure of another of the two or more differentpigments. The two or more different pigments can partially or completelyoverlap with each other. As discussed above, the color perceived by aperson viewing an overlapping region of the two or more differentpigments can depend on a combination of the effects of thereflection/transmission spectra of the different optical structures ofthe two or more different pigments. Some implementations of the securitydevice 90 b can comprise two or more at least partially overlappingfoils, films, threads or laminates comprising different opticalstructures. The color perceived by a person viewing an overlappingregion of the two or more at least partially overlapping foils, films,threads or laminates can depend on a combination of the effects of thereflection/transmission spectra of the different optical structures ofthe two or more foils, films, threads or laminates.

FIG. 12 illustrates a side view of an object 100 with a security devicecomprising a main body 103 of the object and a layer 102 comprising theoptical structure 10, 70 a or 70 b. The object can be a banknote. Themain body may comprise paper comprising the banknote. The layer 102 canbe a laminate, a thread, or a label. When the layer 102 is configured asa label, an adhesive (e.g., a varnish) can be applied to the main body103 and the layer 102 can be adhered to the adhesive of the main body103 using a polymeric adhesive. Alternatively, the adhesive can beapplied to the layer 102 before being affixed to the main body 103. Whenthe layer 102 is configured as a laminate, the layer 102 can be adheredto the main body 103 using a polymer.

The layer 102 can be adhered to the main body 103 using adhesives, suchas, for example optical clear adhesive and/or a cross-linking thermosetadhesive. The security device 100 further comprises a layer 101comprising a message that is composed using a text, a symbol, a numberor any combination thereof that is disposed on the side of the main body(e.g., paper/fabric) 103 of the object (e.g., banknote) opposite theside on which the layer 102 as shown in FIG. 12. Alternately, the layer101 can be disposed between the main body (e.g., paper/fabric) 103 andthe layer 102 or over the layer 102. The layer 101 can comprise, forexample, a dye, a pigment or a phosphorescent material that has the samecolor characteristics as the color reflected or transmitted by theoptical structure 10 when viewed along a direction normal to the surfaceof the layer 102. Accordingly, the message is not visible to an observer(or hidden) when the security device 100 is viewed along a directionnormal to the surface of the layer 102. However, when the securitydevice 100 is tilted such that viewing angle changes, the colorreflected by and/or transmitted through the optical structure 10 changessuch that the message become visible to the observer. In certain cases,the layer 101 comprising a message printed with a phosphorescentmaterial can be made visible when illuminated by UV. The resultant colorof the phosphorescent material can be the combined color of thefluorescence and the dichroic color.

FIG. 13 shows the effect of changing the viewing angle in transmissionof the security device 100 from 0 to about 45 degrees. When the viewingangle is 0 degrees, the message comprising a combination of a number,text or a symbol is not visible in the transmission mode because thecolor of the text is the same as the color of the optical structure intransmission mode (e.g., orange). However, as the viewing angleincreases, the color of the optical structure in transmission modeshifts. For example, the message 203 becomes visible as the color of theoptical structure in transmission mode shifts from orange to yellow asthe angle of observation increases. The color of the message hassufficient contrast with respect to the transmitted color of the opticalstructure 10 so as to be visible to the observer.

In other embodiments, the security device 100 can be configured tooperate in reverse to that described above such that for example themessage is visible at normal incidence and not visible when the securitydevice is tilted. Other variations are possible.

As describe above, the optical structures 10, 70 a or 70 b may be usedin different forms, such as a laminate, a foil, a film, a hot stamp, athread, pigment, ink, or paint. In some implementations, a laminate, afoil, a film, or a thread can comprise a pigment, ink or paintcomprising the optical structures 10, 70 a or 70 b. A laminate may beadhered to a document, product or package using adhesive. A thread maybe threaded or woven through an opening, for example, in the document. Afoil can be hot stamped on the document, product or package. Pigment,ink, or paint may be deposited on the document, product or package orthe material (e.g., paper, cardboard, or fabric) used to form thedocument, product, or package. For example, the document, product, orpackage may be exposed to (e.g., contacted with) the pigment, ink, orpaint to color the document, product or packages in process similar tothose used for non-color shifting pigments, dyes, paints and inks.

A plurality of optical structures 10, 70 a or 70 b such as describedherein collected together as a pigment (as well as inks, and paints) canhave similar optical characteristics as the optical structure 10, 70 aor 70 b configured as a film/laminate. As described above, opticalstructures 10, 70 a or 70 b collected together to form a pigments canexhibit as a collection of platelets or separate pieces the same opticalcharacteristics as the bulk optical film from which the platelets weremade. An added advantage of the optical structures 10, 70 a or 70 bconfigured as a pigment is that color can be blended according todesired specification. The color of the optical structure 10 can bedesigned by using computer software to calculate the thickness of thevarious layers of the optical structure 10, 70 a or 70 b that wouldprovide a desired reflection and/or transmission characteristics.Optical structures 10, 70 a or 70 b that can provide specific colors canbe designed using the computer software and then fabricated.Additionally, different color shifting optical structures 10, 70 a or 70b that produce different colors can be included together and/or colorshifting optical structures such as described herein can be combinedwith non-color shifting pigments or dyes to produce different colors.

The optical structure 10, 70 a or 70 b can be fabricated using a varietyof methods including but not limited to vacuum deposition, coatingmethods, etc. One method of fabrication of the optical structures 10described herein uses a vacuum coater that employs batch or rollcoating. In one method of fabricating the optical structure 10, a firsttransparent high index layer (e.g., layer 12 or layer 16 of FIG. 1) isdeposited onto carrier or base layer such as a sheet or web or othersubstrate. The carrier, web, base layer or substrate can comprisematerials such as, for example, polyester or a polyester with releasecharacteristics such that the optical structure can be readily separatedfrom the web or base layer. A release layer between the base layer andthe plurality of other optical layers the form the optical structure maybe used to permit separation of the optical layers comprising theoptical structure from the base layer or web. A first metal layer (e.g.,layer 13 or layer 15), a transparent dielectric layer comprising high orlow refractive index material (e.g., layer 14), a second metal layer(e.g., layer 15 or layer 13), and a second transparent high index layer(e.g., layer 16 or layer 12) is deposited over the first transparenthigh index layer in sequence (e.g., layer 12 or layer 16 of FIG. 1). Thevarious layers can be deposited in sequence in some embodiments.However, in other embodiments, one or more intervening layers can bedisposed between any of the first metal layer, the transparentdielectric layer comprising high or low refractive index material, thesecond metal layer, and the second transparent high index layer. Asexamples, in some cases the transparent high index layers and thedielectric layer can be deposited using electron gun while the first andthe second metal layers can be deposited by using electron gun orsputtering.

Some materials, like ZnS or MgF2, can be evaporated from a resistancesource. In instances wherein the transparent dielectric layer comprisinghigh or low refractive index material comprises a polymer, a processknown as PML (Polymer Multi-Layer) as described in U.S. Pat. No.5,877,895 can be used. The disclosure of U.S. Pat. No. 5,877,895 isincorporated by reference herein in its entirety.

The disclosure set forth herein describes a wide variety of structuresand method but should not be considered to be limited to thoseparticular structures or methods. For example, although many of theexample optical structures 10 are symmetrical, asymmetric structures arealso possible. For example, instead of having a pair of similar oridentical dielectric layers sandwiching the pair of metallic layers,either dielectric layers having different characteristics (e.g.,thickness or material) may be used on opposite sides of the structure oralternatively, maybe only one side of the pair of metal layers has adielectric layer thereon. Similarly, the metal layers need not beidentical and may have different characteristics such as differentthicknesses or materials. As described above, intervening layers mayalso be included. One or more such intervening layer may be include suchthat the optical structure is not symmetric. For example, an interveninglayer may be included between the dielectric layer 12 and metal layer 13and not between that metal layer 15 and the dielectric layer 16 or viceversa. Similarly, an intervening layer may be included between the metallayer 13 and the dielectric layer 14 and not between the dielectriclayer 14 and the metal layer 15, or vice versa. Similarly, interveninglayer having different characteristics (e.g., material or thickness) maybe included on different sides of the optical structure 10. Or moreintervening layers may be include on one side of the optical structure10 than on the other side of the optical structure. For example, themetal layer 13 and/or the metal layer 15 can comprise metal sub-layers.In some implementations, the metal layer 13 and/or the metal layer 15can comprise a first metal (e.g., silver) facing the high refractiveindex transparent layers 12 or 16 and a second metal (e.g., gold) facingthe dielectric layer 14.

Likewise, although this disclosure describes applications of thestructures and method describe herein to include security applications,e.g., countering successful use of counterfeit currency, documents, andproducts, this disclosure should not be considered to be limited tothose particular applications. Alternatively or in addition, suchfeatures could, for example, be used for providing an aesthetic effect,to create appealing or attractive features on products or packaging formarketing and advertisement, or for other reasons.

Dimensions, such as, thickness, length, width of various embodimentsdescribed herein can be outside the different ranges provided in thisdisclosure. The values of refractive indices for the various materialsdiscussed herein can be outside the different ranges provided in thisdisclosure. The values for reflectance and/or transmittance of thedifferent structures can be outside the different ranges providedherein. The values for spectral widths and peak locations for thereflection and transmission spectra can be outside the different rangesprovided herein.

Various embodiments of the present invention have been described herein.Although this invention has been described with reference to thesespecific embodiments, the descriptions are intended to be illustrativeof the invention and are not intended to be limiting. Variousmodifications and applications may occur to those skilled in the artwithout departing from the true spirit and scope of the invention.

1. An optical structure comprising: a first transparent dielectric layerhaving a refractive index greater than or equal to 1.65; a first metallayer disposed over the first transparent dielectric layer, the firstmetal layer having a first refractive index, wherein a ratio of the realpart (n) of the first refractive index to the imaginary part (k) of thefirst refractive index (k) is greater than or equal to 0.01 and lessthan or equal to 0.5; a second transparent dielectric layer disposedover the first metal layer; a second metal layer disposed over thesecond transparent dielectric layer, the second metal layer having asecond refractive index, wherein a ratio of the real part (n) of thesecond refractive index to the imaginary part (k) of the secondrefractive index is greater than or equal to 0.01 and less than or equalto 0.5; and a third transparent dielectric layer disposed over thesecond metal layer, the third transparent dielectric layer having arefractive index greater than or equal to 1.65.
 2. The optical structureof claim 1, wherein the second transparent dielectric layer has arefractive index less than 1.65.
 3. The optical structure of claim 1,wherein the second transparent dielectric layer has a refractive indexgreater than or equal to 1.65.
 4. The optical structure of claim 1,having a transmission peak comprising: a maximum transmittance greaterthan 50%; and a spectral bandwidth defined by a full width of thetransmission peak at 50% of the maximum transmittance, wherein themaximum transmittance is at least 50%, and wherein the spectralbandwidth of the transmission peak is greater than 2 nm.
 5. The opticalstructure of claim 4, wherein the spectral bandwidth of the transmissionpeak is greater than or equal to about 10 nm and less than or equal toabout 200 nm.
 6. The optical structure of claim 4, wherein the maximumtransmittance is at a wavelength between about 400 nm and about 700 nm.7. The optical structure of claim 4, further comprising a reflectionpeak comprising: a maximum reflectance; and a spectral bandwidth definedby a full width of the reflection peak at 50% of the maximumreflectance, wherein the maximum reflectance is at least 50%, andwherein the spectral bandwidth of the reflection peak is greater than 2nm.
 8. The optical structure of claim 7, wherein the spectral bandwidthof the reflection peak is greater than or equal to about 10 nm and lessthan or equal to about 200 nm.
 9. The optical structure of claim 7,wherein the maximum reflectance is at a wavelength between about 400 nmand about 700 nm.
 10. The optical structure of claim 7, wherein themaximum transmittance is at a first wavelength, and wherein the maximumreflectance is at a second wavelength different from the firstwavelength.
 11. The optical structure of claim 1, configured to displaya first color when viewed by an average human eye along a directionnormal to a surface of the optical structure in reflection mode and asecond color different from the first color when viewed by the averagehuman eye along the direction normal to the surface of the opticalstructure in transmission mode.
 12. The optical structure of claim 11,wherein the first color shifts to a third color when viewed by theaverage human eye along a direction at an angle with respect to thenormal to the surface of the optical structure in reflection mode. 13.The optical structure of claim 11, wherein the second color shifts to afourth color when viewed by the average human eye along a direction atan angle with respect to the normal to the surface of the opticalstructure in transmission mode.
 14. The optical structure of claim 1,wherein the first or the second metal layer has a thickness greater thanor equal to about 5 nm and less than or equal to about 35 nm.
 15. Theoptical structure of claim 1, wherein the second transparent dielectriclayer has a thickness greater than or equal to about 100 nm and lessthan or equal to about 2 microns.
 16. The optical structure of claim 1,wherein first or the third transparent dielectric layer has a thicknessgreater than or equal to about 100 nm and less than or equal to about500 nm. 17.-19. (canceled)
 20. The optical structure of claim 1, whereinthe first or the second metal layer comprises at least one of aluminum,silver, gold, silver alloy, or gold alloy.
 21. (canceled)
 22. Theoptical structure of claim 1, wherein the second transparent dielectriclayer comprises at least one of SiO₂, MgF₂ or a polymer.
 23. The opticalstructure of claim 1, wherein the first or the third transparentdielectric layer comprises at least one of zinc oxide (ZnO), zincsulfide (ZnS), zirconium dioxide (ZrO₂), titanium dioxide (TiO₂),tantalum pentoxide (Ta₂O₅), ceric oxide (CeO₂), ytterium oxide (Y₂O₃),indium oxide (In₂O₃), tin oxide (SnO₂), indium tin oxide (ITO), tungstentrioxide (WO₃), or combinations thereof. 24.-26. (canceled)
 27. Theoptical structure of claim 1, configured as a film, a pigment, a paintor an ink. 28.-39. (canceled)
 40. A banknote or a document comprisingthe optical structure of claim
 1. 41.-124. (canceled)
 125. The opticalstructure of claim 3, configured to display a first color in reflectionmode when viewed by an average human eye at a viewing angle between 0degrees and 40 degrees with respect to a normal to a surface of theoptical structure and a second color different from the first color intransmission mode when viewed by the average human eye at the viewingangle between 0 degrees and 40 degrees with respect to the normal to thesurface of the optical structure.
 126. The optical structure of claim125, wherein the first color appears substantially the same to theaverage human eye at viewing angles between 0 degrees and 40 degreeswith respect to the normal to the surface of the optical structure. 127.The optical structure of claim 125, wherein the second color appearssubstantially the same to the average human eye at viewing anglesbetween 0 degrees and 40 degrees with respect to the normal to thesurface of the optical structure.